Fix race issues among panic, NMI and crash_kexec

If panic on NMI happens just after panic() on the same CPU, panic() is recursively called. As the result, it stalls after failing to acquire panic_lock.

To avoid this problem, don't call panic() in NMI context if we've already entered panic().

V4:
- Improve comments in io_check_error() and panic()

V3:
- Introduce nmi_panic() macro to reduce code duplication
- In the case of panic on NMI, don't return from NMI handlers if another cpu already panicked

V2:
- Use atomic_cmpxchg() instead of current spin_trylock() to exclude concurrent accesses to the panic routines
- Don't introduce no-lock version of panic()

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Michal Hocko <[email protected]>

---
 arch/x86/kernel/nmi.c  |   16 ++++++++++++----
 include/linux/kernel.h |   13 +++++++++++++
 kernel/panic.c         |   15 ++++++++++++---
 kernel/watchdog.c      |    2 +-
 4 files changed, 38 insertions(+), 8 deletions(-)

arch/x86/kernel/nmi.c +1 -1

index 697f90d..5131714 100644
@@ -231,7 +231,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
 #endif
 
 	if (panic_on_unrecovered_nmi)
-		panic("NMI: Not continuing");
+		nmi_panic("NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 

arch/x86/kernel/nmi.c +9 -1

+
+		/*
+		 * If we return from nmi_panic(), it means we have received
+		 * NMI while processing panic().  So, simply return without
+		 * a delay and re-enabling NMI.
+		 */
+		return;
+	}
 
 	/* Re-enable the IOCK line, wait for a few seconds */
 	reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;

@@ -297,7 +305,7 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 
 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
-		panic("NMI: Not continuing");
+		nmi_panic("NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 }

include/linux/kernel.h +0 -0

index 350dfb0..480a4fd 100644

include/linux/kernel.h +12 -0

+
+/*
+ * A variant of panic() called from NMI context.
+ * If we've already panicked on this cpu, return from here.
+ */
+#define nmi_panic(fmt, ...)						\
+	do {								\
+		int this_cpu = raw_smp_processor_id();			\
+		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
+			panic(fmt, ##__VA_ARGS__);			\
+	} while (0)
+
 /*
  * Only to be used by arch init code. If the user over-wrote the default
  * CONFIG_PANIC_TIMEOUT, honor it.

kernel/panic.c +4 -2

index 4579dbb..24ee2ea 100644
@@ -61,6 +61,8 @@ void __weak panic_smp_self_stop(void)
 		cpu_relax();
 }
 
+atomic_t panic_cpu = ATOMIC_INIT(-1);
+
 /**
  *	panic - halt the system
  *	@fmt: The text string to print

@@ -71,17 +73,17 @@ void __weak panic_smp_self_stop(void)
  */
 void panic(const char *fmt, ...)
 {
-	static DEFINE_SPINLOCK(panic_lock);
 	static char buf[1024];
 	va_list args;
 	long i, i_next = 0;
 	int state = 0;
+	int old_cpu, this_cpu;
 
 	/*
 	 * Disable local interrupts. This will prevent panic_smp_self_stop
 	 * from deadlocking the first cpu that invokes the panic, since
 	 * there is nothing to prevent an interrupt handler (that runs
-	 * after the panic_lock is acquired) from invoking panic again.
+	 * after setting panic_cpu) from invoking panic again.
 	 */
 	local_irq_disable();
 

Quoted 51 lines

If panic on NMI happens just after panic() on the same CPU, panic() is recursively called. As the result, it stalls after failing to acquire panic_lock.

To avoid this problem, don't call panic() in NMI context if we've already entered panic().

V4:
- Improve comments in io_check_error() and panic()

V3:
- Introduce nmi_panic() macro to reduce code duplication
- In the case of panic on NMI, don't return from NMI handlers if another cpu already panicked

V2:
- Use atomic_cmpxchg() instead of current spin_trylock() to exclude concurrent accesses to the panic routines
- Don't introduce no-lock version of panic()

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Michal Hocko <[email protected]>

---
 arch/x86/kernel/nmi.c  |   16 ++++++++++++----
 include/linux/kernel.h |   13 +++++++++++++
 kernel/panic.c         |   15 ++++++++++++---
 kernel/watchdog.c      |    2 +-
 4 files changed, 38 insertions(+), 8 deletions(-)

arch/x86/kernel/nmi.c +1 -1

index 697f90d..5131714 100644
@@ -231,7 +231,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
 #endif
 
 	if (panic_on_unrecovered_nmi)
-		panic("NMI: Not continuing");
+		nmi_panic("NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 

@@ -255,8 +255,16 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
 		 reason, smp_processor_id());
 	show_regs(regs);
 

arch/x86/kernel/nmi.c io_check_error(unsigned char reason, struct pt_regs *regs) · line 259

Quoted 4 lines

arch/x86/kernel/nmi.c +2 -2

-	if (panic_on_io_nmi)
-		panic("NMI IOCK error: Not continuing");
+	if (panic_on_io_nmi) {
+		nmi_panic("NMI IOCK error: Not continuing");

Btw, that panic_on_io_nmi seems undocumented in Documentation/sysctl/kernel.txt. Care to document it, please, as a separate patch?

Quoted 26 lines

arch/x86/kernel/nmi.c +9 -1

+
+		/*
+		 * If we return from nmi_panic(), it means we have received
+		 * NMI while processing panic().  So, simply return without
+		 * a delay and re-enabling NMI.
+		 */
+		return;
+	}
 
 	/* Re-enable the IOCK line, wait for a few seconds */
 	reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;

@@ -297,7 +305,7 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 
 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
-		panic("NMI: Not continuing");
+		nmi_panic("NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 }

include/linux/kernel.h +0 -0

index 350dfb0..480a4fd 100644
@@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
 

include/linux/kernel.h extern int sysctl_panic_on_stackoverflow; · line 448

Quoted 3 lines

include/linux/kernel.h +1 -0

 extern bool crash_kexec_post_notifiers;
 
+extern atomic_t panic_cpu;

This needs a comment explaining what this variable is and what it denotes.

Quoted 50 lines

include/linux/kernel.h +12 -0

+
+/*
+ * A variant of panic() called from NMI context.
+ * If we've already panicked on this cpu, return from here.
+ */
+#define nmi_panic(fmt, ...)						\
+	do {								\
+		int this_cpu = raw_smp_processor_id();			\
+		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
+			panic(fmt, ##__VA_ARGS__);			\
+	} while (0)
+
 /*
  * Only to be used by arch init code. If the user over-wrote the default
  * CONFIG_PANIC_TIMEOUT, honor it.

kernel/panic.c +4 -2

index 4579dbb..24ee2ea 100644
@@ -61,6 +61,8 @@ void __weak panic_smp_self_stop(void)
 		cpu_relax();
 }
 
+atomic_t panic_cpu = ATOMIC_INIT(-1);
+
 /**
  *	panic - halt the system
  *	@fmt: The text string to print

@@ -71,17 +73,17 @@ void __weak panic_smp_self_stop(void)
  */
 void panic(const char *fmt, ...)
 {
-	static DEFINE_SPINLOCK(panic_lock);
 	static char buf[1024];
 	va_list args;
 	long i, i_next = 0;
 	int state = 0;
+	int old_cpu, this_cpu;
 
 	/*
 	 * Disable local interrupts. This will prevent panic_smp_self_stop
 	 * from deadlocking the first cpu that invokes the panic, since
 	 * there is nothing to prevent an interrupt handler (that runs
-	 * after the panic_lock is acquired) from invoking panic again.
+	 * after setting panic_cpu) from invoking panic again.
 	 */
 	local_irq_disable();
 

kernel/panic.c void panic(const char *fmt, ...) · line 103

Quoted 8 lines

kernel/panic.c +5 -0

@@ -94,8 +96,15 @@ void panic(const char *fmt, ...)
 	 * multiple parallel invocations of panic, all other CPUs either
 	 * stop themself or will wait until they are stopped by the 1st CPU
 	 * with smp_send_stop().
+	 *
+	 * `old_cpu == -1' means this is the 1st CPU which comes here, so
+	 * go ahead.
+	 * `old_cpu == this_cpu' means we came from nmi_panic() which sets
+	 * panic_cpu to this cpu.  In this case, this is also the 1st CPU.

I'd prefer that -1 to be

#define INVALID_CPU_NUM -1

or

#define UNDEFINED_CPU_NUM -1

or so instead of a naked number.

nmi_shootdown_cpus(), a subroutine of crash_kexec(), sends NMI IPI to non-panic cpus to stop them while saving their register

information and doing some cleanups for crash dumping. So if a non-panic cpus is infinitely looping in NMI context, we fail to

save its register information and lose the information from the crash dump.

`Infinite loop in NMI context' can happen:

a. when a cpu panics on NMI while another cpu is processing panic b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI

In the case of a, it loops in panic_smp_self_stop(). In the case of b, it loops in raw_spin_lock() of nmi_reason_lock.

This can
happen on some servers which broadcasts NMIs to all CPUs when a dump button is pushed.

To save registers in these case too, this patch does following things:

1. Move the timing of `infinite loop in NMI context' (actually done by panic_smp_self_stop()) outside of panic() to enable us to refer pt_regs

2. call a callback of nmi_shootdown_cpus() directly to save registers and do some cleanups after setting waiting_for_crash_ipi which is used for counting down the number of cpus which handled the callback

V5:
- Use WRITE_ONCE() when setting crash_ipi_done to 1 so that the compiler doesn't change the instruction order
- Support the case of b in the above description
- Add poll_crash_ipi_and_callback()

V4:
- Rewrite the patch description

V3:
- Newly introduced

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Vivek Goyal <[email protected]>

Cc Michal Hocko <[email protected]>

---
 arch/x86/include/asm/reboot.h |    1 +
 arch/x86/kernel/nmi.c         |   17 +++++++++++++----
 arch/x86/kernel/reboot.c      |   28 ++++++++++++++++++++++++++++
 include/linux/kernel.h        |   12 ++++++++++--
 kernel/panic.c                |   10 ++++++++++
 kernel/watchdog.c             |    2 +-
 6 files changed, 63 insertions(+), 7 deletions(-)

arch/x86/include/asm/reboot.h +1 -0

index a82c4f1..964e82f 100644
@@ -25,5 +25,6 @@ void __noreturn machine_real_restart(unsigned int type);
 
 typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);
 void nmi_shootdown_cpus(nmi_shootdown_cb callback);
+void poll_crash_ipi_and_callback(struct pt_regs *regs);
 
 #endif /* _ASM_X86_REBOOT_H */

arch/x86/kernel/nmi.c +4 -3

index 5131714..74a1434 100644
@@ -29,6 +29,7 @@
 #include <asm/mach_traps.h>
 #include <asm/nmi.h>
 #include <asm/x86_init.h>
+#include <asm/reboot.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/nmi.h>

@@ -231,7 +232,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
 #endif
 
 	if (panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 

@@ -256,7 +257,7 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
 	show_regs(regs);
 
 	if (panic_on_io_nmi) {
-		nmi_panic("NMI IOCK error: Not continuing");
+		nmi_panic(regs, "NMI IOCK error: Not continuing");
 
 		/*
 		 * If we return from nmi_panic(), it means we have received

@@ -305,7 +306,7 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 
 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 }

arch/x86/kernel/nmi.c +1 -0

+
 	reason = x86_platform.get_nmi_reason();
 
 	if (reason & NMI_REASON_MASK) {

arch/x86/kernel/reboot.c +4 -0

index 02693dd..44c5f5b 100644
@@ -718,6 +718,7 @@ static int crashing_cpu;
 static nmi_shootdown_cb shootdown_callback;
 
 static atomic_t waiting_for_crash_ipi;
+static int crash_ipi_done;
 
 static int crash_nmi_callback(unsigned int val, struct pt_regs *regs)
 {

@@ -780,6 +781,9 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 
 	smp_send_nmi_allbutself();
 
+	/* Kick cpus looping in nmi context. */
+	WRITE_ONCE(crash_ipi_done, 1);
+
 	msecs = 1000; /* Wait at most a second for the other cpus to stop */
 	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
 		mdelay(1);

arch/x86/kernel/reboot.c +18 -0

+{
+	if (crash_ipi_done)
+		crash_nmi_callback(0, regs); /* Shouldn't return */
+}
+
+/* Override the weak function in kernel/panic.c */
+void nmi_panic_self_stop(struct pt_regs *regs)
+{
+	while (1) {
+		poll_crash_ipi_and_callback(regs);
+		cpu_relax();
+	}
+}
+
 #else /* !CONFIG_SMP */
 void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 {
 	/* No other CPUs to shoot down */
 }
+
+void poll_crash_ipi_and_callback(struct pt_regs *regs)
+{
+}
 #endif

include/linux/kernel.h +4 -1

index 480a4fd..728a31b 100644
@@ -255,6 +255,7 @@ extern long (*panic_blink)(int state);
 __printf(1, 2)
 void panic(const char *fmt, ...)
 	__noreturn __cold;
+void nmi_panic_self_stop(struct pt_regs *);
 extern void oops_enter(void);
 extern void oops_exit(void);
 void print_oops_end_marker(void);

@@ -450,12 +451,19 @@ extern atomic_t panic_cpu;
 /*
  * A variant of panic() called from NMI context.
  * If we've already panicked on this cpu, return from here.
+ * If another cpu already panicked, loop in nmi_panic_self_stop() which
+ * can provide architecture dependent code such as saving register states
+ * for crash dump.
  */
-#define nmi_panic(fmt, ...)						\

kernel/panic.c +8 -0

+ * Architecture code may override this to prepare for crash dumping
+ * (e.g. save register information).
+ */
+void __weak nmi_panic_self_stop(struct pt_regs *regs)
+{
+	panic_smp_self_stop();
+}
+
 atomic_t panic_cpu = ATOMIC_INIT(-1);
 
 /**

kernel/watchdog.c +1 -1

index b9be18f..84b5035 100644
@@ -351,7 +351,7 @@ static void watchdog_overflow_callback(struct perf_event *event,
 			trigger_allbutself_cpu_backtrace();
 
 		if (hardlockup_panic)
-			nmi_panic("Hard LOCKUP");
+			nmi_panic(regs, "Hard LOCKUP");
 
 		__this_cpu_write(hard_watchdog_warn, true);
 		return;

If panic on NMI happens just after panic() on the same CPU, panic() is recursively called. As the result, it stalls after failing to acquire panic_lock.

To avoid this problem, don't call panic() in NMI context if we've already entered panic().

V4:
- Improve comments in io_check_error() and panic()

V3:
- Introduce nmi_panic() macro to reduce code duplication
- In the case of panic on NMI, don't return from NMI handlers if another cpu already panicked

V2:
- Use atomic_cmpxchg() instead of current spin_trylock() to exclude concurrent accesses to the panic routines
- Don't introduce no-lock version of panic()

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Michal Hocko <[email protected]>

---
 arch/x86/kernel/nmi.c  |   16 ++++++++++++----
 include/linux/kernel.h |   13 +++++++++++++
 kernel/panic.c         |   15 ++++++++++++---
 kernel/watchdog.c      |    2 +-
 4 files changed, 38 insertions(+), 8 deletions(-)

arch/x86/kernel/nmi.c +12 -4

index 697f90d..5131714 100644
@@ -231,7 +231,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
 #endif
 
 	if (panic_on_unrecovered_nmi)
-		panic("NMI: Not continuing");
+		nmi_panic("NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 

@@ -255,8 +255,16 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
 		 reason, smp_processor_id());
 	show_regs(regs);
 
-	if (panic_on_io_nmi)
-		panic("NMI IOCK error: Not continuing");
+	if (panic_on_io_nmi) {
+		nmi_panic("NMI IOCK error: Not continuing");
+
+		/*
+		 * If we return from nmi_panic(), it means we have received
+		 * NMI while processing panic().  So, simply return without
+		 * a delay and re-enabling NMI.
+		 */
+		return;
+	}
 
 	/* Re-enable the IOCK line, wait for a few seconds */
 	reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;

@@ -297,7 +305,7 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 
 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
-		panic("NMI: Not continuing");
+		nmi_panic("NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 }

include/linux/kernel.h +13 -0

index 350dfb0..480a4fd 100644
@@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
 
 extern bool crash_kexec_post_notifiers;
 
+extern atomic_t panic_cpu;
+
+/*
+ * A variant of panic() called from NMI context.
+ * If we've already panicked on this cpu, return from here.
+ */
+#define nmi_panic(fmt, ...)						\
+	do {								\
+		int this_cpu = raw_smp_processor_id();			\
+		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
+			panic(fmt, ##__VA_ARGS__);			\
+	} while (0)
+
 /*
  * Only to be used by arch init code. If the user over-wrote the default
  * CONFIG_PANIC_TIMEOUT, honor it.

kernel/panic.c +12 -3

index 4579dbb..24ee2ea 100644
@@ -61,6 +61,8 @@ void __weak panic_smp_self_stop(void)
 		cpu_relax();
 }
 
+atomic_t panic_cpu = ATOMIC_INIT(-1);
+
 /**
  *	panic - halt the system
  *	@fmt: The text string to print

@@ -71,17 +73,17 @@ void __weak panic_smp_self_stop(void)
  */
 void panic(const char *fmt, ...)
 {
-	static DEFINE_SPINLOCK(panic_lock);
 	static char buf[1024];
 	va_list args;
 	long i, i_next = 0;
 	int state = 0;
+	int old_cpu, this_cpu;
 
 	/*
 	 * Disable local interrupts. This will prevent panic_smp_self_stop
 	 * from deadlocking the first cpu that invokes the panic, since
 	 * there is nothing to prevent an interrupt handler (that runs
-	 * after the panic_lock is acquired) from invoking panic again.
+	 * after setting panic_cpu) from invoking panic again.
 	 */
 	local_irq_disable();
 

@@ -94,8 +96,15 @@ void panic(const char *fmt, ...)
 	 * multiple parallel invocations of panic, all other CPUs either
 	 * stop themself or will wait until they are stopped by the 1st CPU
 	 * with smp_send_stop().
+	 *
+	 * `old_cpu == -1' means this is the 1st CPU which comes here, so
+	 * go ahead.
+	 * `old_cpu == this_cpu' means we came from nmi_panic() which sets
+	 * panic_cpu to this cpu.  In this case, this is also the 1st CPU.
 	 */
-	if (!spin_trylock(&panic_lock))
+	this_cpu = raw_smp_processor_id();
+	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
+	if (old_cpu != -1 && old_cpu != this_cpu)
 		panic_smp_self_stop();
 
 	console_verbose();

kernel/watchdog.c +1 -1

index 18f34cf..b9be18f 100644
@@ -351,7 +351,7 @@ static void watchdog_overflow_callback(struct perf_event *event,
 			trigger_allbutself_cpu_backtrace();
 
 		if (hardlockup_panic)
-			panic("Hard LOCKUP");
+			nmi_panic("Hard LOCKUP");
 
 		__this_cpu_write(hard_watchdog_warn, true);
 		return;

nmi_shootdown_cpus(), a subroutine of crash_kexec(), sends NMI IPI to non-panic cpus to stop them while saving their register information and doing some cleanups for crash dumping. So if a non-panic cpus is infinitely looping in NMI context, we fail to save its register information and lose the information from the crash dump.

`Infinite loop in NMI context' can happen:

a. when a cpu panics on NMI while another cpu is processing panic b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI

In the case of a, it loops in panic_smp_self_stop(). In the case of b, it loops in raw_spin_lock() of nmi_reason_lock. This can happen on some servers which broadcasts NMIs to all CPUs when a dump button is pushed.

To save registers in these case too, this patch does following things:

1. Move the timing of `infinite loop in NMI context' (actually done by panic_smp_self_stop()) outside of panic() to enable us to refer pt_regs
2. call a callback of nmi_shootdown_cpus() directly to save registers and do some cleanups after setting waiting_for_crash_ipi which is used for counting down the number of cpus which handled the callback

V5:
- Use WRITE_ONCE() when setting crash_ipi_done to 1 so that the compiler doesn't change the instruction order
- Support the case of b in the above description
- Add poll_crash_ipi_and_callback()

V4:
- Rewrite the patch description

V3:
- Newly introduced

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Vivek Goyal <[email protected]>

Cc Michal Hocko <[email protected]>

---
 arch/x86/include/asm/reboot.h |    1 +
 arch/x86/kernel/nmi.c         |   17 +++++++++++++----
 arch/x86/kernel/reboot.c      |   28 ++++++++++++++++++++++++++++
 include/linux/kernel.h        |   12 ++++++++++--
 kernel/panic.c                |   10 ++++++++++
 kernel/watchdog.c             |    2 +-
 6 files changed, 63 insertions(+), 7 deletions(-)

arch/x86/include/asm/reboot.h +1 -0

index a82c4f1..964e82f 100644
@@ -25,5 +25,6 @@ void __noreturn machine_real_restart(unsigned int type);
 
 typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);
 void nmi_shootdown_cpus(nmi_shootdown_cb callback);
+void poll_crash_ipi_and_callback(struct pt_regs *regs);
 
 #endif /* _ASM_X86_REBOOT_H */

arch/x86/kernel/nmi.c +13 -4

index 5131714..74a1434 100644
@@ -29,6 +29,7 @@
 #include <asm/mach_traps.h>
 #include <asm/nmi.h>
 #include <asm/x86_init.h>
+#include <asm/reboot.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/nmi.h>

@@ -231,7 +232,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
 #endif
 
 	if (panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 

@@ -256,7 +257,7 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
 	show_regs(regs);
 
 	if (panic_on_io_nmi) {
-		nmi_panic("NMI IOCK error: Not continuing");
+		nmi_panic(regs, "NMI IOCK error: Not continuing");
 
 		/*
 		 * If we return from nmi_panic(), it means we have received

@@ -305,7 +306,7 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 
 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 }

@@ -357,7 +358,15 @@ static void default_do_nmi(struct pt_regs *regs)
 	}
 
 	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
-	raw_spin_lock(&nmi_reason_lock);
+
+	/*
+	 * Another CPU may be processing panic routines with holding
+	 * nmi_reason_lock.  Check IPI issuance from the panicking CPU
+	 * and call the callback directly.
+	 */
+	while (!raw_spin_trylock(&nmi_reason_lock))
+		poll_crash_ipi_and_callback(regs);
+
 	reason = x86_platform.get_nmi_reason();
 
 	if (reason & NMI_REASON_MASK) {

arch/x86/kernel/reboot.c +28 -0

index 02693dd..44c5f5b 100644
@@ -718,6 +718,7 @@ static int crashing_cpu;
 static nmi_shootdown_cb shootdown_callback;
 
 static atomic_t waiting_for_crash_ipi;
+static int crash_ipi_done;
 
 static int crash_nmi_callback(unsigned int val, struct pt_regs *regs)
 {

@@ -780,6 +781,9 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 
 	smp_send_nmi_allbutself();
 
+	/* Kick cpus looping in nmi context. */
+	WRITE_ONCE(crash_ipi_done, 1);
+
 	msecs = 1000; /* Wait at most a second for the other cpus to stop */
 	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
 		mdelay(1);

@@ -788,9 +792,33 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 
 	/* Leave the nmi callback set */
 }
+
+/*
+ * Wait for the timing of IPI for crash dumping, and then call its callback
+ * directly.  This function is used when we have already been in NMI handler.
+ */
+void poll_crash_ipi_and_callback(struct pt_regs *regs)
+{
+	if (crash_ipi_done)
+		crash_nmi_callback(0, regs); /* Shouldn't return */
+}
+
+/* Override the weak function in kernel/panic.c */
+void nmi_panic_self_stop(struct pt_regs *regs)
+{
+	while (1) {
+		poll_crash_ipi_and_callback(regs);
+		cpu_relax();
+	}
+}
+
 #else /* !CONFIG_SMP */
 void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 {
 	/* No other CPUs to shoot down */
 }
+
+void poll_crash_ipi_and_callback(struct pt_regs *regs)
+{
+}
 #endif

include/linux/kernel.h +10 -2

index 480a4fd..728a31b 100644
@@ -255,6 +255,7 @@ extern long (*panic_blink)(int state);
 __printf(1, 2)
 void panic(const char *fmt, ...)
 	__noreturn __cold;
+void nmi_panic_self_stop(struct pt_regs *);
 extern void oops_enter(void);
 extern void oops_exit(void);
 void print_oops_end_marker(void);

@@ -450,12 +451,19 @@ extern atomic_t panic_cpu;
 /*
  * A variant of panic() called from NMI context.
  * If we've already panicked on this cpu, return from here.
+ * If another cpu already panicked, loop in nmi_panic_self_stop() which
+ * can provide architecture dependent code such as saving register states
+ * for crash dump.
  */
-#define nmi_panic(fmt, ...)						\
+#define nmi_panic(regs, fmt, ...)					\
 	do {								\
+		int old_cpu;						\
 		int this_cpu = raw_smp_processor_id();			\
-		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
+		old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);	\
+		if (old_cpu == -1)					\
 			panic(fmt, ##__VA_ARGS__);			\
+		else if (old_cpu != this_cpu)				\
+			nmi_panic_self_stop(regs);			\
 	} while (0)
 
 /*

kernel/panic.c +10 -0

index 24ee2ea..4fce2be 100644
@@ -61,6 +61,16 @@ void __weak panic_smp_self_stop(void)
 		cpu_relax();
 }
 
+/*
+ * Stop ourself in NMI context if another cpu has already panicked.
+ * Architecture code may override this to prepare for crash dumping
+ * (e.g. save register information).
+ */
+void __weak nmi_panic_self_stop(struct pt_regs *regs)
+{
+	panic_smp_self_stop();
+}
+
 atomic_t panic_cpu = ATOMIC_INIT(-1);
 
 /**

kernel/watchdog.c +1 -1

index b9be18f..84b5035 100644
@@ -351,7 +351,7 @@ static void watchdog_overflow_callback(struct perf_event *event,
 			trigger_allbutself_cpu_backtrace();
 
 		if (hardlockup_panic)
-			nmi_panic("Hard LOCKUP");
+			nmi_panic(regs, "Hard LOCKUP");
 
 		__this_cpu_write(hard_watchdog_warn, true);
 		return;

Currently, panic() and crash_kexec() can be called at the same time. For example (x86 case):

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
nmi_shootdown_cpus() // stop other cpus

CPU 1:
panic()
crash_kexec()
mutex_trylock() // failed to acquire
smp_send_stop() // stop other cpus
infinite loop

If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump fails.

In another case:

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
<NMI>
io_check_error()
panic()
crash_kexec()
mutex_trylock() // failed to acquire
infinite loop

Clearly, this is an undesirable result.

To fix this problem, this patch changes crash_kexec() to exclude others by using atomic_t panic_cpu.

V5:
- Add missing dummy __crash_kexec() for !CONFIG_KEXEC_CORE case
- Replace atomic_xchg() with atomic_set() in crash_kexec() because it is used as a release operation and there is no need of memory barrier effect. This change also removes an unused value warning

V4:
- Use new __crash_kexec(), no exclusion check version of crash_kexec(), instead of checking if panic_cpu is the current cpu or not

V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Vivek Goyal <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Michal Hocko <[email protected]>

kernel/kexec_core.c +10 -0

+void crash_kexec(struct pt_regs *regs)
+{
+	int old_cpu, this_cpu;
+
+	/*
+	 * Only one CPU is allowed to execute the crash_kexec() code as with
+	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
+	 * may stop each other.  To exclude them, we use panic_cpu here too.
+	 */
+	this_cpu = raw_smp_processor_id();

cmpxchg(&panic_cpu, -1, 0) != 0

returns -1 for cpu 0, thus 0 != 0, and sets panic_cpu to 0

include/linux/kernel.h +1 -0

index 350dfb0..480a4fd 100644
@@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
 
 extern bool crash_kexec_post_notifiers;
 
+extern atomic_t panic_cpu;

include/linux/kernel.h +2 -0

+	} while (0)
+
 /*
  * Only to be used by arch init code. If the user over-wrote the default
  * CONFIG_PANIC_TIMEOUT, honor it.

kernel/panic.c +0 -0

index 4579dbb..24ee2ea 100644

arch/x86/kernel/nmi.c static void default_do_nmi(struct pt_regs *regs) · line 368

Quoted 4 lines

arch/x86/kernel/nmi.c +3 -0

+	 */
+	while (!raw_spin_trylock(&nmi_reason_lock))
+		poll_crash_ipi_and_callback(regs);

Waaait a minute: so if we're getting NMIs broadcasted on every core but we're *not* crash dumping, we will run into here too. This can't be right. :-\

Quoted 8 lines · Borislav Petkov

arch/x86/kernel/nmi.c static void default_do_nmi(struct pt_regs *regs) · line 368

Quoted 4 lines

arch/x86/kernel/nmi.c +3 -0

+	 */
+	while (!raw_spin_trylock(&nmi_reason_lock))
+		poll_crash_ipi_and_callback(regs);

Waaait a minute: so if we're getting NMIs broadcasted on every core but we're *not* crash dumping, we will run into here too. This can't be right. :-\

This only does something if crash_ipi_done is set, which means you are killing the box.

But perhaps a comment that states that here would be useful, or maybe just put in the check here. There's no need to make it depend on SMP, as raw_spin_trylock() will turn to just ({1}) for UP, and that code wont even be hit.

Quoted 9 lines · Hidehiro Kawai

include/linux/kernel.h +1 -0

index 350dfb0..480a4fd 100644
@@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
 
 extern bool crash_kexec_post_notifiers;
 
+extern atomic_t panic_cpu;

include/linux/kernel.h extern int sysctl_panic_on_stackoverflow; · line 458

Quoted 10 lines

include/linux/kernel.h +10 -0

+
+/*
+ * A variant of panic() called from NMI context.
+ * If we've already panicked on this cpu, return from here.
+ */
+#define nmi_panic(fmt, ...)						\
+	do {								\
+		int this_cpu = raw_smp_processor_id();			\
+		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
+			panic(fmt, ##__VA_ARGS__);			\

Hmm,

What happens if:

	CPU 0:				CPU 1:
	------				------
	nmi_panic();

					nmi_panic();
					<external nmi>
					nmi_panic();

Currently, panic() and crash_kexec() can be called at the same time. For example (x86 case):

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
nmi_shootdown_cpus() // stop other cpus

CPU 1:
panic()
crash_kexec()
mutex_trylock() // failed to acquire
smp_send_stop() // stop other cpus
infinite loop

If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump fails.

In another case:

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
<NMI>
io_check_error()
panic()
crash_kexec()
mutex_trylock() // failed to acquire
infinite loop

Clearly, this is an undesirable result.

To fix this problem, this patch changes crash_kexec() to exclude others by using atomic_t panic_cpu.

V5:
- Add missing dummy __crash_kexec() for !CONFIG_KEXEC_CORE case
- Replace atomic_xchg() with atomic_set() in crash_kexec() because it is used as a release operation and there is no need of memory barrier effect. This change also removes an unused value warning

V4:
- Use new __crash_kexec(), no exclusion check version of crash_kexec(), instead of checking if panic_cpu is the current cpu or not

V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Vivek Goyal <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Michal Hocko <[email protected]>

---
 include/linux/kexec.h |    2 ++
 kernel/kexec_core.c   |   26 +++++++++++++++++++++++++-
 kernel/panic.c        |    4 ++--
 3 files changed, 29 insertions(+), 3 deletions(-)

include/linux/kexec.h +2 -0

index d140b1e..7b68d27 100644
@@ -237,6 +237,7 @@ extern int kexec_purgatory_get_set_symbol(struct kimage *image,
 					  unsigned int size, bool get_value);
 extern void *kexec_purgatory_get_symbol_addr(struct kimage *image,
 					     const char *name);
+extern void __crash_kexec(struct pt_regs *);
 extern void crash_kexec(struct pt_regs *);
 int kexec_should_crash(struct task_struct *);
 void crash_save_cpu(struct pt_regs *regs, int cpu);

@@ -332,6 +333,7 @@ int __weak arch_kexec_apply_relocations(const Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 #else /* !CONFIG_KEXEC_CORE */
 struct pt_regs;
 struct task_struct;
+static inline void __crash_kexec(struct pt_regs *regs) { }
 static inline void crash_kexec(struct pt_regs *regs) { }
 static inline int kexec_should_crash(struct task_struct *p) { return 0; }
 #define kexec_in_progress false

kernel/kexec_core.c +25 -1

index 11b64a6..9d097f5 100644
@@ -853,7 +853,8 @@ struct kimage *kexec_image;
 struct kimage *kexec_crash_image;
 int kexec_load_disabled;
 
-void crash_kexec(struct pt_regs *regs)
+/* No panic_cpu check version of crash_kexec */
+void __crash_kexec(struct pt_regs *regs)
 {
 	/* Take the kexec_mutex here to prevent sys_kexec_load
 	 * running on one cpu from replacing the crash kernel

@@ -876,6 +877,29 @@ void crash_kexec(struct pt_regs *regs)
 	}
 }
 
+void crash_kexec(struct pt_regs *regs)
+{
+	int old_cpu, this_cpu;
+
+	/*
+	 * Only one CPU is allowed to execute the crash_kexec() code as with
+	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
+	 * may stop each other.  To exclude them, we use panic_cpu here too.
+	 */
+	this_cpu = raw_smp_processor_id();
+	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
+	if (old_cpu == -1) {
+		/* This is the 1st CPU which comes here, so go ahead. */
+		__crash_kexec(regs);
+
+		/*
+		 * Reset panic_cpu to allow another panic()/crash_kexec()
+		 * call.
+		 */
+		atomic_set(&panic_cpu, -1);
+	}
+}
+
 size_t crash_get_memory_size(void)
 {
 	size_t size = 0;

kernel/panic.c +0 -0

index 4fce2be..5d0b807 100644

kernel/panic.c +0 -0

 	bust_spinlocks(0);
 
 

Quoted 6 lines

What happens if:

	CPU 0:				CPU 1:
	------				------
	nmi_panic();

Quoted 3 lines

					nmi_panic();
					<external nmi>
					nmi_panic();  

I thought that nmi_panic is called only from the nmi context. If so how can we get a nested NMI like that?

Quoted 2 lines

nmi_shootdown_cpus(), a subroutine of crash_kexec(), sends NMI IPI to non-panic cpus to stop them while saving their register

...to stop them and save their register...

Quoted 2 lines

information and doing some cleanups for crash dumping. So if a non-panic cpus is infinitely looping in NMI context, we fail to

That should be CPU. Please use "CPU" instead of "cpu" in all your text in your next submission.

Quoted 9 lines

save its register information and lose the information from the crash dump.

`Infinite loop in NMI context' can happen:

a. when a cpu panics on NMI while another cpu is processing panic b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI

Quoted 2 lines

In the case of a, it loops in panic_smp_self_stop(). In the case of b, it loops in raw_spin_lock() of nmi_reason_lock.

Please describe those two cases more verbosely - it takes slow people like me a while to figure out what exactly can happen.

Quoted 4 lines

This can
happen on some servers which broadcasts NMIs to all CPUs when a dump button is pushed.

Quoted 5 lines

To save registers in these case too, this patch does following things:

1. Move the timing of `infinite loop in NMI context' (actually done by panic_smp_self_stop()) outside of panic() to enable us to refer pt_regs

I can't parse that sentence. And I really tried :-\
panic_smp_self_stop() is still in panic().

Quoted 88 lines

2. call a callback of nmi_shootdown_cpus() directly to save registers and do some cleanups after setting waiting_for_crash_ipi which is used for counting down the number of cpus which handled the callback

V5:
- Use WRITE_ONCE() when setting crash_ipi_done to 1 so that the compiler doesn't change the instruction order
- Support the case of b in the above description
- Add poll_crash_ipi_and_callback()

V4:
- Rewrite the patch description

V3:
- Newly introduced

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Vivek Goyal <[email protected]>

Cc Michal Hocko <[email protected]>

---
 arch/x86/include/asm/reboot.h |    1 +
 arch/x86/kernel/nmi.c         |   17 +++++++++++++----
 arch/x86/kernel/reboot.c      |   28 ++++++++++++++++++++++++++++
 include/linux/kernel.h        |   12 ++++++++++--
 kernel/panic.c                |   10 ++++++++++
 kernel/watchdog.c             |    2 +-
 6 files changed, 63 insertions(+), 7 deletions(-)

arch/x86/include/asm/reboot.h +1 -0

index a82c4f1..964e82f 100644
@@ -25,5 +25,6 @@ void __noreturn machine_real_restart(unsigned int type);
 
 typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);
 void nmi_shootdown_cpus(nmi_shootdown_cb callback);
+void poll_crash_ipi_and_callback(struct pt_regs *regs);
 
 #endif /* _ASM_X86_REBOOT_H */

arch/x86/kernel/nmi.c +4 -3

index 5131714..74a1434 100644
@@ -29,6 +29,7 @@
 #include <asm/mach_traps.h>
 #include <asm/nmi.h>
 #include <asm/x86_init.h>
+#include <asm/reboot.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/nmi.h>

@@ -231,7 +232,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
 #endif
 
 	if (panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 

@@ -256,7 +257,7 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
 	show_regs(regs);
 
 	if (panic_on_io_nmi) {
-		nmi_panic("NMI IOCK error: Not continuing");
+		nmi_panic(regs, "NMI IOCK error: Not continuing");
 
 		/*
 		 * If we return from nmi_panic(), it means we have received

@@ -305,7 +306,7 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 
 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 }

@@ -357,7 +358,15 @@ static void default_do_nmi(struct pt_regs *regs)
 	}
 

arch/x86/kernel/nmi.c static void default_do_nmi(struct pt_regs *regs) · line 363

Quoted 5 lines

arch/x86/kernel/nmi.c +3 -1

 	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
-	raw_spin_lock(&nmi_reason_lock);
+
+	/*
+	 * Another CPU may be processing panic routines with holding

while

arch/x86/kernel/nmi.c static void default_do_nmi(struct pt_regs *regs) · line 365

Quoted 2 lines

arch/x86/kernel/nmi.c +2 -0

+	 * nmi_reason_lock.  Check IPI issuance from the panicking CPU
+	 * and call the callback directly.

This is one strange sentence. Does it mean something like:

"Check if the panicking CPU issued the IPI and, if so, call the crash callback directly."

?

arch/x86/kernel/nmi.c static void default_do_nmi(struct pt_regs *regs) · line 368

Quoted 3 lines

arch/x86/kernel/nmi.c +3 -0

+	 */
+	while (!raw_spin_trylock(&nmi_reason_lock))
+		poll_crash_ipi_and_callback(regs);

Waaait a minute: so if we're getting NMIs broadcasted on every core but we're *not* crash dumping, we will run into here too. This can't be right. :-\

Quoted 28 lines

arch/x86/kernel/nmi.c +1 -0

+
 	reason = x86_platform.get_nmi_reason();
 
 	if (reason & NMI_REASON_MASK) {

arch/x86/kernel/reboot.c +4 -0

index 02693dd..44c5f5b 100644
@@ -718,6 +718,7 @@ static int crashing_cpu;
 static nmi_shootdown_cb shootdown_callback;
 
 static atomic_t waiting_for_crash_ipi;
+static int crash_ipi_done;
 
 static int crash_nmi_callback(unsigned int val, struct pt_regs *regs)
 {

@@ -780,6 +781,9 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 
 	smp_send_nmi_allbutself();
 
+	/* Kick cpus looping in nmi context. */
+	WRITE_ONCE(crash_ipi_done, 1);
+
 	msecs = 1000; /* Wait at most a second for the other cpus to stop */
 	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
 		mdelay(1);

@@ -788,9 +792,33 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 

arch/x86/kernel/reboot.c void nmi_shootdown_cpus(nmi_shootdown_cb callback) · line 797

Quoted 5 lines

arch/x86/kernel/reboot.c +3 -0

 	/* Leave the nmi callback set */
 }
+
+/*
+ * Wait for the timing of IPI for crash dumping, and then call its callback

"Wait for the crash dumping IPI to complete... "

arch/x86/kernel/reboot.c void nmi_shootdown_cpus(nmi_shootdown_cb callback) · line 800

Quoted 3 lines

arch/x86/kernel/reboot.c +3 -0

+ * directly.  This function is used when we have already been in NMI handler.
+ */
+void poll_crash_ipi_and_callback(struct pt_regs *regs)

Why "poll"? We won't return from crash_nmi_callback() if we're not the crashing CPU.

Quoted 45 lines

arch/x86/kernel/reboot.c +18 -0

+{
+	if (crash_ipi_done)
+		crash_nmi_callback(0, regs); /* Shouldn't return */
+}
+
+/* Override the weak function in kernel/panic.c */
+void nmi_panic_self_stop(struct pt_regs *regs)
+{
+	while (1) {
+		poll_crash_ipi_and_callback(regs);
+		cpu_relax();
+	}
+}
+
 #else /* !CONFIG_SMP */
 void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 {
 	/* No other CPUs to shoot down */
 }
+
+void poll_crash_ipi_and_callback(struct pt_regs *regs)
+{
+}
 #endif

include/linux/kernel.h +4 -1

index 480a4fd..728a31b 100644
@@ -255,6 +255,7 @@ extern long (*panic_blink)(int state);
 __printf(1, 2)
 void panic(const char *fmt, ...)
 	__noreturn __cold;
+void nmi_panic_self_stop(struct pt_regs *);
 extern void oops_enter(void);
 extern void oops_exit(void);
 void print_oops_end_marker(void);

@@ -450,12 +451,19 @@ extern atomic_t panic_cpu;
 /*
  * A variant of panic() called from NMI context.
  * If we've already panicked on this cpu, return from here.
+ * If another cpu already panicked, loop in nmi_panic_self_stop() which
+ * can provide architecture dependent code such as saving register states
+ * for crash dump.
  */
-#define nmi_panic(fmt, ...)						\

include/linux/kernel.h extern atomic_t panic_cpu; · line 466

Quoted 10 lines

include/linux/kernel.h +6 -1

+#define nmi_panic(regs, fmt, ...)					\
 	do {								\
+		int old_cpu;						\
 		int this_cpu = raw_smp_processor_id();			\
-		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
+		old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);	\
+		if (old_cpu == -1)					\
 			panic(fmt, ##__VA_ARGS__);			\
+		else if (old_cpu != this_cpu)				\
+			nmi_panic_self_stop(regs);			\

Same here - this is assuming that broadcasting NMIs to all cores automatically means there's a crash dump in progress:

nmi_panic_self_stop() -> poll_crash_ipi_and_callback()

and this cannot be right.

Quoted 11 lines

include/linux/kernel.h +0 -0

 	} while (0)
 
 /*

kernel/panic.c +0 -0

index 24ee2ea..4fce2be 100644
@@ -61,6 +61,16 @@ void __weak panic_smp_self_stop(void)
 		cpu_relax();
 }
 

kernel/panic.c void __weak panic_smp_self_stop(void) · line 65

Quoted 2 lines

kernel/panic.c +2 -0

+/*
+ * Stop ourself in NMI context if another cpu has already panicked.

"ourselves"

Quoted 8 lines

arch/x86/kernel/nmi.c static void default_do_nmi(struct pt_regs *regs) · line 368

Quoted 4 lines

arch/x86/kernel/nmi.c +3 -0

+	 */
+	while (!raw_spin_trylock(&nmi_reason_lock))
+		poll_crash_ipi_and_callback(regs);

Waaait a minute: so if we're getting NMIs broadcasted on every core but we're *not* crash dumping, we will run into here too. This can't be right. :-\

This only does something if crash_ipi_done is set, which means you are killing the box. But perhaps a comment that states that here would be useful, or maybe just put in the check here.

There's no need to make it depend on SMP, as
raw_spin_trylock() will turn to just ({1}) for UP, and that code wont even be hit.

Quoted 16 lines

Currently, panic() and crash_kexec() can be called at the same time. For example (x86 case):

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
nmi_shootdown_cpus() // stop other cpus

CPU 1:
panic()
crash_kexec()
mutex_trylock() // failed to acquire
smp_send_stop() // stop other cpus
infinite loop

Quoted 2 lines

If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump fails.

So the smp_send_stop() stops CPU 0 from calling nmi_shootdown_cpus(), right?

Quoted 14 lines

In another case:

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
<NMI>
io_check_error()
panic()
crash_kexec()
mutex_trylock() // failed to acquire
infinite loop

Quoted 1 line

Clearly, this is an undesirable result.

I'm trying to see how this patch fixes this case.

Quoted 99 lines

To fix this problem, this patch changes crash_kexec() to exclude others by using atomic_t panic_cpu.

V5:
- Add missing dummy __crash_kexec() for !CONFIG_KEXEC_CORE case
- Replace atomic_xchg() with atomic_set() in crash_kexec() because it is used as a release operation and there is no need of memory barrier effect. This change also removes an unused value warning

V4:
- Use new __crash_kexec(), no exclusion check version of crash_kexec(), instead of checking if panic_cpu is the current cpu or not

V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Vivek Goyal <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Michal Hocko <[email protected]>

---
 include/linux/kexec.h |    2 ++
 kernel/kexec_core.c   |   26 +++++++++++++++++++++++++-
 kernel/panic.c        |    4 ++--
 3 files changed, 29 insertions(+), 3 deletions(-)

include/linux/kexec.h +2 -0

index d140b1e..7b68d27 100644
@@ -237,6 +237,7 @@ extern int kexec_purgatory_get_set_symbol(struct kimage *image,
 					  unsigned int size, bool get_value);
 extern void *kexec_purgatory_get_symbol_addr(struct kimage *image,
 					     const char *name);
+extern void __crash_kexec(struct pt_regs *);
 extern void crash_kexec(struct pt_regs *);
 int kexec_should_crash(struct task_struct *);
 void crash_save_cpu(struct pt_regs *regs, int cpu);

@@ -332,6 +333,7 @@ int __weak arch_kexec_apply_relocations(const Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 #else /* !CONFIG_KEXEC_CORE */
 struct pt_regs;
 struct task_struct;
+static inline void __crash_kexec(struct pt_regs *regs) { }
 static inline void crash_kexec(struct pt_regs *regs) { }
 static inline int kexec_should_crash(struct task_struct *p) { return 0; }
 #define kexec_in_progress false

kernel/kexec_core.c +25 -1

index 11b64a6..9d097f5 100644
@@ -853,7 +853,8 @@ struct kimage *kexec_image;
 struct kimage *kexec_crash_image;
 int kexec_load_disabled;
 
-void crash_kexec(struct pt_regs *regs)
+/* No panic_cpu check version of crash_kexec */
+void __crash_kexec(struct pt_regs *regs)
 {
 	/* Take the kexec_mutex here to prevent sys_kexec_load
 	 * running on one cpu from replacing the crash kernel

@@ -876,6 +877,29 @@ void crash_kexec(struct pt_regs *regs)
 	}
 }
 
+void crash_kexec(struct pt_regs *regs)
+{
+	int old_cpu, this_cpu;
+
+	/*
+	 * Only one CPU is allowed to execute the crash_kexec() code as with
+	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
+	 * may stop each other.  To exclude them, we use panic_cpu here too.
+	 */
+	this_cpu = raw_smp_processor_id();
+	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
+	if (old_cpu == -1) {
+		/* This is the 1st CPU which comes here, so go ahead. */
+		__crash_kexec(regs);
+
+		/*
+		 * Reset panic_cpu to allow another panic()/crash_kexec()
+		 * call.
+		 */
+		atomic_set(&panic_cpu, -1);
+	}
+}
+
 size_t crash_get_memory_size(void)
 {
 	size_t size = 0;

kernel/panic.c +0 -0

index 4fce2be..5d0b807 100644
@@ -138,7 +138,7 @@ void panic(const char *fmt, ...)

kernel/panic.c void panic(const char *fmt, ...) · line 141

Quoted 5 lines

kernel/panic.c +1 -1

 	 * the "crash_kexec_post_notifiers" option to the kernel.
 	 */
 	if (!crash_kexec_post_notifiers)
-		crash_kexec(NULL);
+		__crash_kexec(NULL);

Why call the __crash_kexec() version and not just crash_kexec() here. This needs to be documented.

Quoted 11 lines

Quoted 8 lines

`Infinite loop in NMI context' can happen:

a. when a cpu panics on NMI while another cpu is processing panic b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI

In the case of a, it loops in panic_smp_self_stop(). In the case of b, it loops in raw_spin_lock() of nmi_reason_lock.

Please describe those two cases more verbosely - it takes slow people like me a while to figure out what exactly can happen.

a. when a cpu panics on NMI while another cpu is processing panic

     Ex.
     CPU 0                     CPU 1
     =================         =================
     panic()
       panic_cpu <-- 0
       check panic_cpu
       crash_kexec()
                               receive an unknown NMI
                               unknown_nmi_error()
                                 nmi_panic()
                                   panic()
                                     check panic_cpu
                                     panic_smp_self_stop()
                                       infinite loop in NMI context

b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI
Ex.
CPU 0 CPU 1
====================== ==================
receive an unknown NMI
unknown_nmi_error()
nmi_panic() receive an unknown NMI
panic_cpu <-- 0 unknown_nmi_error()
panic() nmi_panic()
check panic_cpu panic
crash_kexec() check panic_cpu
panic_smp_self_stop()
infinite loop in NMI context

Quoted 8 lines

Quoted 5 lines

To save registers in these case too, this patch does following things:

1. Move the timing of `infinite loop in NMI context' (actually done by panic_smp_self_stop()) outside of panic() to enable us to refer pt_regs

I can't parse that sentence. And I really tried :-\
panic_smp_self_stop() is still in panic().

panic_smp_self_stop() is still used when a CPU in normal context should go into infinite loop. Only when a CPU is in NMI context, nmi_panic_self_stop() is called and the CPU loops infinitely without entering panic().

I'll try to revise this sentense.

Quoted 7 lines

arch/x86/kernel/nmi.c static void default_do_nmi(struct pt_regs *regs) · line 368

Quoted 3 lines

arch/x86/kernel/nmi.c +3 -0

+	 */
+	while (!raw_spin_trylock(&nmi_reason_lock))
+		poll_crash_ipi_and_callback(regs);

Waaait a minute: so if we're getting NMIs broadcasted on every core but we're *not* crash dumping, we will run into here too. This can't be right. :-\

As Steven commented, poll_crash_ipi_and_callback() does nothing if we're not crash dumping. But yes, this is confusing. I'll add more detailed comment, or change the logic a bit if I come up with better one.

Quoted 4 lines

Quoted 2 lines

+/*
+ * Wait for the timing of IPI for crash dumping, and then call its callback

"Wait for the crash dumping IPI to complete... "

So, I think "Wait for the crash dumping IPI to be issued..." is better. "complete" would be ambiguous in this context.

Quoted 7 lines

Quoted 4 lines

+ * directly. This function is used when we have already been in NMI handler.
+ */
+void poll_crash_ipi_and_callback(struct pt_regs *regs)

Why "poll"? We won't return from crash_nmi_callback() if we're not the crashing CPU.

This function polls that crash IPI has been issued by checking crash_ipi_done, then invokes the callback. This is different from so-called "poll" functions. Do you have some good name?

Quoted 28 lines

Quoted 11 lines

Quoted 8 lines

`Infinite loop in NMI context' can happen:

a. when a cpu panics on NMI while another cpu is processing panic b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI

In the case of a, it loops in panic_smp_self_stop(). In the case of b, it loops in raw_spin_lock() of nmi_reason_lock.

Please describe those two cases more verbosely - it takes slow people like me a while to figure out what exactly can happen.

a. when a cpu panics on NMI while another cpu is processing panic

     Ex.
     CPU 0                     CPU 1
     =================         =================
     panic()
       panic_cpu <-- 0
       check panic_cpu
       crash_kexec()
                               receive an unknown NMI
                               unknown_nmi_error()
                                 nmi_panic()
                                   panic()
                                     check panic_cpu
                                     panic_smp_self_stop()
                                       infinite loop in NMI context

Quoted 14 lines

b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI
Ex.
CPU 0 CPU 1
====================== ==================
receive an unknown NMI
unknown_nmi_error()
nmi_panic() receive an unknown NMI
panic_cpu <-- 0 unknown_nmi_error()
panic() nmi_panic()
check panic_cpu panic
crash_kexec() check panic_cpu
panic_smp_self_stop()
infinite loop in NMI context

Ok, that's what I saw too, thanks for confirming.

But please write those examples with the *old* code in the commit message, i.e. without panic_cpu and nmi_panic() which you're adding.

Basically, you want to structure your commit message this way:

This is the problem the current code has: ...

But we need to do this: ...

We fix it by doing that: ...

Quoted 4 lines

Quoted 4 lines

Quoted 3 lines

+ * directly. This function is used when we have already been in NMI handler.
+ */
+void poll_crash_ipi_and_callback(struct pt_regs *regs)

Quoted 6 lines

Quoted 2 lines

Why "poll"? We won't return from crash_nmi_callback() if we're not the crashing CPU.

This function polls that crash IPI has been issued by checking crash_ipi_done, then invokes the callback. This is different from so-called "poll" functions. Do you have some good name?

Maybe something as simple as "run_crash_callback"?

Or since we're calling it from other places, maybe add the "crash" prefix:

	while (!raw_spin_trylock(&nmi_reason_lock))
		crash_run_callback(regs);

Looks even better to me in code context. :)

Currently, panic() and crash_kexec() can be called at the same time. For example (x86 case):

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
nmi_shootdown_cpus() // stop other cpus

CPU 1:
panic()
crash_kexec()
mutex_trylock() // failed to acquire
smp_send_stop() // stop other cpus
infinite loop

If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump fails.

In another case:

CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
<NMI>
io_check_error()
panic()
crash_kexec()
mutex_trylock() // failed to acquire
infinite loop

Clearly, this is an undesirable result.

To fix this problem, this patch changes crash_kexec() to exclude others by using atomic_t panic_cpu.

V5:
- Add missing dummy __crash_kexec() for !CONFIG_KEXEC_CORE case
- Replace atomic_xchg() with atomic_set() in crash_kexec() because it is used as a release operation and there is no need of memory barrier effect. This change also removes an unused value warning

V4:
- Use new __crash_kexec(), no exclusion check version of crash_kexec(), instead of checking if panic_cpu is the current cpu or not

V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Vivek Goyal <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Michal Hocko <[email protected]>

---
 include/linux/kexec.h |    2 ++
 kernel/kexec_core.c   |   26 +++++++++++++++++++++++++-
 kernel/panic.c        |    4 ++--
 3 files changed, 29 insertions(+), 3 deletions(-)

kernel/kexec_core.c +8 -0

+void crash_kexec(struct pt_regs *regs)
+{
+	int old_cpu, this_cpu;
+
+	/*
+	 * Only one CPU is allowed to execute the crash_kexec() code as with
+	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
+	 * may stop each other.  To exclude them, we use panic_cpu here too.

Does *old* code mean the code without this patch *series* ?

I prefer this, but we might want to add some more prefix or suffix. For example, "conditionally_run_crash_nmi_callback".

Thanks for your deep review!

This patch introduces new boot option, apic_extnmi:

apic_extnmi={ bsp | all | none}

The default value is "bsp" and this is the current behavior; only BSP receives external NMI. "all" allows external NMIs to be broadcast to all CPUs. This would raise the success rate of panic on NMI when BSP hangs up in NMI context or the external NMI is swallowed by other NMI handlers on BSP. If you specified "none", any CPUs don't receive external NMIs. This is useful for dump capture kernel so that it wouldn't be shot down while saving a crash dump.

V5:
- Rename the option from "noextnmi" to "apic_extnmi"
- Add apic_extnmi=all feature
- Fix the wrong documentation about "noextnmi" (apic_extnmi=none)

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Jonathan Corbet <[email protected]>

---
 Documentation/kernel-parameters.txt |    9 +++++++++
 arch/x86/include/asm/apic.h         |    5 +++++
 arch/x86/kernel/apic/apic.c         |   31 ++++++++++++++++++++++++++++++-
 3 files changed, 44 insertions(+), 1 deletion(-)

Documentation/kernel-parameters.txt +0 -0

index f8aae63..ceed3bc 100644

Documentation/kernel-parameters.txt +6 -0

+			all:  External NMIs are broadcast to all CPUs as a
+			      backup of CPU 0
+			none: External NMI is masked for all CPUs. This is
+			      useful so that a dump capture kernel won't be
+			      shot down by NMI
+
 	autoconf=	[IPV6]
 			See Documentation/networking/ipv6.txt.
 

arch/x86/include/asm/apic.h +5 -0

index 7f62ad4..c80f6b6 100644
@@ -23,6 +23,11 @@
 #define APIC_VERBOSE 1
 #define APIC_DEBUG   2
 
+/* Macros for apic_extnmi which controls external NMI masking */
+#define APIC_EXTNMI_BSP		0 /* Default */
+#define APIC_EXTNMI_ALL		1
+#define APIC_EXTNMI_NONE	2
+
 /*
  * Define the default level of output to be very little
  * This can be turned up by using apic=verbose for more

arch/x86/kernel/apic/apic.c +0 -0

index 2f69e3b..a2a8074 100644

arch/x86/kernel/apic/apic.c +6 -0

+ */
+static int apic_extnmi = APIC_EXTNMI_BSP;
+
+/*
  * Map cpu index to physical APIC ID
  */
 DEFINE_EARLY_PER_CPU_READ_MOSTLY(u16, x86_cpu_to_apicid, BAD_APICID);

@@ -1161,6 +1167,8 @@ void __init init_bsp_APIC(void)
 	value = APIC_DM_NMI;
 	if (!lapic_is_integrated())		/* 82489DX */
 		value |= APIC_LVT_LEVEL_TRIGGER;
+	if (apic_extnmi == APIC_EXTNMI_NONE)
+		value |= APIC_LVT_MASKED;
 	apic_write(APIC_LVT1, value);
 }
 

arch/x86/kernel/apic/apic.c +2 -1

-	if (!cpu)
+	if ((!cpu && apic_extnmi != APIC_EXTNMI_NONE) ||
+	    apic_extnmi == APIC_EXTNMI_ALL)
 		value = APIC_DM_NMI;
 	else
 		value = APIC_DM_NMI | APIC_LVT_MASKED;

Quoted 2 lines

I prefer this, but we might want to add some more prefix or suffix. For example, "conditionally_run_crash_nmi_callback".

That's unnecessary IMO. If you need to express that, you could write that in a comment above the function definition. Anyone who looks at the code then will know that it is conditional, like so many other kernel functions. :)

Quoted 37 lines

This patch introduces new boot option, apic_extnmi:

apic_extnmi={ bsp | all | none}

The default value is "bsp" and this is the current behavior; only BSP receives external NMI. "all" allows external NMIs to be broadcast to all CPUs. This would raise the success rate of panic on NMI when BSP hangs up in NMI context or the external NMI is swallowed by other NMI handlers on BSP. If you specified "none", any CPUs don't receive external NMIs. This is useful for dump capture kernel so that it wouldn't be shot down while saving a crash dump.

V5:
- Rename the option from "noextnmi" to "apic_extnmi"
- Add apic_extnmi=all feature
- Fix the wrong documentation about "noextnmi" (apic_extnmi=none)

Signed-off-by Hidehiro Kawai <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc "H. Peter Anvin" <[email protected]>

Cc Jonathan Corbet <[email protected]>

---
 Documentation/kernel-parameters.txt |    9 +++++++++
 arch/x86/include/asm/apic.h         |    5 +++++
 arch/x86/kernel/apic/apic.c         |   31 ++++++++++++++++++++++++++++++-
 3 files changed, 44 insertions(+), 1 deletion(-)

Documentation/kernel-parameters.txt +0 -0

index f8aae63..ceed3bc 100644
@@ -472,6 +472,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Change the amount of debugging information output
 			when initialising the APIC and IO-APIC components.
 

Documentation/kernel-parameters.txt bytes respectively. Such letter suffixes can also be entirely omitted. · line 477

Quoted 3 lines

Documentation/kernel-parameters.txt +3 -0

+	apic_extnmi=	[APIC,X86] External NMI delivery setting
+			Format: { bsp (default) | all | none }
+			bsp:  External NMI is delivered to only CPU 0

only to

Quoted 33 lines

Documentation/kernel-parameters.txt +6 -0

+			all:  External NMIs are broadcast to all CPUs as a
+			      backup of CPU 0
+			none: External NMI is masked for all CPUs. This is
+			      useful so that a dump capture kernel won't be
+			      shot down by NMI
+
 	autoconf=	[IPV6]
 			See Documentation/networking/ipv6.txt.
 

arch/x86/include/asm/apic.h +5 -0

index 7f62ad4..c80f6b6 100644
@@ -23,6 +23,11 @@
 #define APIC_VERBOSE 1
 #define APIC_DEBUG   2
 
+/* Macros for apic_extnmi which controls external NMI masking */
+#define APIC_EXTNMI_BSP		0 /* Default */
+#define APIC_EXTNMI_ALL		1
+#define APIC_EXTNMI_NONE	2
+
 /*
  * Define the default level of output to be very little
  * This can be turned up by using apic=verbose for more

arch/x86/kernel/apic/apic.c +0 -0

index 2f69e3b..a2a8074 100644
@@ -82,6 +82,12 @@ physid_mask_t phys_cpu_present_map;
 static unsigned int disabled_cpu_apicid __read_mostly = BAD_APICID;
 

arch/x86/kernel/apic/apic.c physid_mask_t phys_cpu_present_map; · line 86

Quoted 3 lines

arch/x86/kernel/apic/apic.c +2 -0

 /*
+ * This variable controls which CPUs receive external NMIs.  By default,
+ * external NMIs are delivered to only BSP.

only to the BSP.

Quoted 17 lines

arch/x86/kernel/apic/apic.c +6 -0

+ */
+static int apic_extnmi = APIC_EXTNMI_BSP;
+
+/*
  * Map cpu index to physical APIC ID
  */
 DEFINE_EARLY_PER_CPU_READ_MOSTLY(u16, x86_cpu_to_apicid, BAD_APICID);

@@ -1161,6 +1167,8 @@ void __init init_bsp_APIC(void)
 	value = APIC_DM_NMI;
 	if (!lapic_is_integrated())		/* 82489DX */
 		value |= APIC_LVT_LEVEL_TRIGGER;
+	if (apic_extnmi == APIC_EXTNMI_NONE)
+		value |= APIC_LVT_MASKED;
 	apic_write(APIC_LVT1, value);
 }
 

arch/x86/kernel/apic/apic.c void setup_local_APIC(void) · line 1390

Quoted 4 lines

arch/x86/kernel/apic/apic.c +0 -0

@@ -1380,7 +1388,8 @@ void setup_local_APIC(void)
 	/*
 	 * only the BP should see the LINT1 NMI signal, obviously.
 	 */

That comment needs adjusting.

Quoted 8 lines

arch/x86/kernel/apic/apic.c +2 -1

-	if (!cpu)
+	if ((!cpu && apic_extnmi != APIC_EXTNMI_NONE) ||
+	    apic_extnmi == APIC_EXTNMI_ALL)
 		value = APIC_DM_NMI;
 	else
 		value = APIC_DM_NMI | APIC_LVT_MASKED;

arch/x86/kernel/apic/apic.c static int __init apic_set_disabled_cpu_apicid(char *arg) · line 2566

Quoted 10 lines

arch/x86/kernel/apic/apic.c +7 -0

@@ -2548,3 +2557,23 @@ static int __init apic_set_disabled_cpu_apicid(char *arg)
 	return 0;
 }
 early_param("disable_cpu_apicid", apic_set_disabled_cpu_apicid);
+
+static int __init apic_set_extnmi(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	if (strcmp("all", arg) == 0)

	if (!strncmp("all", arg, 3))

ditto for the rest

arch/x86/kernel/apic/apic.c static int __init apic_set_disabled_cpu_apicid(char *arg) · line 2573

Quoted 7 lines

arch/x86/kernel/apic/apic.c +7 -0

+		apic_extnmi = APIC_EXTNMI_ALL;
+	else if (strcmp("none", arg) == 0)
+		apic_extnmi = APIC_EXTNMI_NONE;
+	else if (strcmp("bsp", arg) == 0)
+		apic_extnmi = APIC_EXTNMI_BSP;
+	else {
+		pr_warn("Unknown external NMI delivery mode `%s' is ignored\n",

s/is //

Also, if there's no other delivery mode which makes sense, you can do:

	pr_warn("Unknown external NMI delivery mode `%s', defaulting to	'bsp'\n", arg);
	apic_extnmi = APIC_EXTNMI_BSP;

Btw, you can let the pr_warn line be longer than 80 cols.

And if you don't default, you need

		return -EINVAL;

here.

Quoted 8 lines

kernel/kexec_core.c +8 -0

+void crash_kexec(struct pt_regs *regs)
+{
+	int old_cpu, this_cpu;
+
+	/*
+	 * Only one CPU is allowed to execute the crash_kexec() code as with
+	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
+	 * may stop each other.  To exclude them, we use panic_cpu here too.

kernel/kexec_core.c void crash_kexec(struct pt_regs *regs) · line 897

Quoted 10 lines

kernel/kexec_core.c +10 -0

+	 */
+	this_cpu = raw_smp_processor_id();
+	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
+	if (old_cpu == -1) {
+		/* This is the 1st CPU which comes here, so go ahead. */
+		__crash_kexec(regs);
+
+		/*
+		 * Reset panic_cpu to allow another panic()/crash_kexec()
+		 * call.

So can we make __crash_kexec() return error values?

* failed to grab kexec_mutex -> reset panic_cpu

* no kexec_crash_image -> no need to reset it, all future crash_kexec() calls won't work so no need to run into that path anymore. However, this could be problematic if we want the other CPUs to panic. Do we care?

* machine_kexec successful -> doesn't matter

We can do so, but I think resetting panic_cpu always would be simpler and safer.

Quoted 2 lines

We can do so, but I think resetting panic_cpu always would be simpler and safer.

Well, I think executing code needlessly *especially* at panic time is not all that rosy either.

Besides something like this:

	static bool kexec_failed;

	...

        if (crash_kexec_post_notifiers && !kexec_failed)
		kexec_failed = __crash_kexec(NULL);

is as simple as it gets.

arch/x86/kernel/nmi.c +0 -1

@@ -357,7 +358,15 @@ static void default_do_nmi(struct pt_regs *regs)
 	}
 
 	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
-	raw_spin_lock(&nmi_reason_lock);

Quoted 3 lines · 河合英宏 / KAWAI，HIDEHIRO

Quoted 2 lines

We can do so, but I think resetting panic_cpu always would be simpler and safer.

I'll state in detail.

When we call crash_kexec() without entering panic() and return from it, panic() should be called eventually.

But the code paths are a bit complicated and there are many implementations for each architecture. So one day, this assumption may be broken; the CPU doesn't call panic(). Or the CPU may fail to call panic() because we are already in insane state. It would be nervous, but allowing another CPU to process panic routines by resetting panic_cpu is safer approach.

Since this code is executed only once due to panic_cpu, I think introducing this logic is not much valuable. Also, current implementation is already quite simple:

panic()
{
...

	__crash_kexec(NULL) {
		if (mutex_trylock(&kexec_mutex)) {
			if (kexec_crash_image) {
				/* don't return */
			}

Quoted 8 lines

Quoted 3 lines · 河合英宏 / KAWAI，HIDEHIRO

Quoted 2 lines

We can do so, but I think resetting panic_cpu always would be simpler and safer.

I'll state in detail.

When we call crash_kexec() without entering panic() and return from it, panic() should be called eventually.

Huh, the call chain is

panic->crash_kexec

Or do you mean, when crash_kexec() is not called by panic() but by some of its other callers?

Quoted 6 lines

But the code paths are a bit complicated and there are many implementations for each architecture. So one day, this assumption may be broken; the CPU doesn't call panic(). Or the CPU may fail to call panic() because we are already in insane state. It would be nervous, but allowing another CPU to process panic routines by resetting panic_cpu is safer approach.

My suggestion was to do this only on the panic path - not necessarily on the others.

Quoted 4 lines

Since this code is executed only once due to panic_cpu, I think introducing this logic is not much valuable. Also, current implementation is already quite simple:

Quoted 8 lines

panic()
{
...

	__crash_kexec(NULL) {
		if (mutex_trylock(&kexec_mutex)) {
			if (kexec_crash_image) {
				/* don't return */
			}

I don't mean the kexec_crash_image case - I mean the opposite one:
!kexec_crash_image.

And I think I know now what you're trying to tell
me: the first CPU which hits panic, will finish panic eventually and so it will take down the machine.

I was arguing about the case of oops_end --> crash_kexec --> return from crash_kexec because of !kexec_crash_image --> panic.

In the case of panic --> __crash_kexec, __crash_kexec is called only once, so we don't need to check the return value of __crash_kexec as you suggested. So I thought you stated about crash_kexec --> panic case.

I also mentioned !kexec_crash_image case...

No. The first CPU calls panic, and then it calls __crash_kexec. Because of !kexec_crash_image, it returns from __crash_kexec and continues to the panic procedure. At the same time, another CPU tries to call panic(), but it doesn't run the panic procedure; panic_cpu prevents the second CPU from running it.

This means __crash_kexec is called only once even if we don't check the return value of __crash_kexec.

(Please note that crash_kexec can be called multiple times in the case of oops_end() --> crash_kexec().)