x86_64: support poll() on /dev/mcelog

From: Tim Hockin <[email protected]>

Background:
/dev/mcelog is typically polled manually. This is less than optimal for some situations. Calling poll() on /dev/mcelog does not work.

Description:
This patch adds support for poll() to /dev/mcelog. This results in immediate wakeup of user apps whenever the poller finds MCEs. Because the exception handler can not take any locks, it can not call the wakeup itself. Instead, it uses a thread_info flag (TIF_MCE_NOTIFY) which is caught at the next return from interrupt, calling the mce_user_notify() routine.

This patch also does some small cleanup for essentially unused variables, and moves the user notification into the body of the poller, so it is only called once, rather than once per CPU.

Result:
Applications can now poll() on /dev/mcelog. When an error is logged (whether through the poller or through an exception) the applications are woken up promptly. This should not affect any previous behaviors. If no MCEs are being logged, there is no overhead.

Alternatives:
I considered simply supporting poll() through the poller and not using TIF_MCE_NOTIFY at all. However, the time between an uncorrectable error happening and the user application being notified is *the*most* critical window for us. Many uncorrectable errors can be logged to the network if given a chance.

Testing:
I used an error-injecting DIMM to create lots of correctable DRAM errors and verified that my user app is woken up in sync with the polling interval. I also used the northbridge to inject uncorrectable ECC errors, and verified (printk() to the rescue) that the notify routine is called and the user app does wake up.

Patch:
This patch is against 2.6.21-rc7.

Signed-off-by Tim Hockin <[email protected]>

---

This is the first public version version of this patch. The TIF_* approach was suggested by Mike Waychison and Andi did not yell at me when I suggested it. :)

diff -pruN linux-2.6.20+th/arch/x86_64/kernel/entry.S linux-2.6.20+th2v2/arch/x86_64/kernel/entry.S

arch/x86_64/kernel/entry.S 2007-04-29 18:10:40.000000000 -0700 renamed from arch/x86_64/kernel/entry.S 2007-04-24 22:46:19.000000000 -0700 +18 -2

@@ -585,7 +585,7 @@ bad_iret:
 retint_careful:
        CFI_RESTORE_STATE
        bt    $TIF_NEED_RESCHED,%edx
-       jnc   retint_signal
+       jnc   retint_mce
        TRACE_IRQS_ON
        sti
        pushq %rdi

@@ -597,7 +597,23 @@ retint_careful:
        cli
        TRACE_IRQS_OFF
        jmp retint_check
-
+
+       /* handle a waiting machine check */
+retint_mce:
+       bt $TIF_MCE_NOTIFY,%edx
+       jnc retint_signal
+       TRACE_IRQS_ON
+       sti
+       pushq %rdi
+       CFI_ADJUST_CFA_OFFSET   8
+       call mce_notify_user
+       popq %rdi
+       CFI_ADJUST_CFA_OFFSET   -8
+       GET_THREAD_INFO(%rcx)
+       cli
+       TRACE_IRQS_OFF
+       jmp retint_check
+
 retint_signal:
        testl $(_TIF_SIGPENDING|_TIF_NOTIFY_RESUME|_TIF_SINGLESTEP),%edx
        jz    retint_swapgs

diff -pruN linux-2.6.20+th/arch/x86_64/kernel/mce.c linux-2.6.20+th2v2/arch/x86_64/kernel/mce.c

arch/x86_64/kernel/mce.c 2007-04-29 18:27:22.000000000 -0700 renamed from arch/x86_64/kernel/mce.c 2007-04-27 14:19:08.000000000 -0700 +36 -24

@@ -20,6 +20,8 @@
 #include <linux/percpu.h>
 #include <linux/ctype.h>
 #include <linux/kmod.h>
+#include <linux/poll.h>
+#include <linux/thread_info.h>
 #include <asm/processor.h>
 #include <asm/msr.h>
 #include <asm/mce.h>

@@ -39,8 +41,7 @@ static int mce_dont_init;
 static int tolerant = 1;
 static int banks;
 static unsigned long bank[NR_BANKS] = { [0 ... NR_BANKS-1] = ~0UL };
-static unsigned long console_logged;
-static int notify_user;
+static unsigned long notify_user;
 static int rip_msr;
 static int mce_bootlog = 1;
 static atomic_t mce_events;

@@ -48,6 +49,8 @@ static atomic_t mce_events;
 static char trigger[128];
 static char *trigger_argv[2] = { trigger, NULL };
 
+static DECLARE_WAIT_QUEUE_HEAD(mce_wait);
+
 /*
  * Lockless MCE logging infrastructure.
  * This avoids deadlocks on printk locks without having to break locks. Also

@@ -94,8 +97,7 @@ void mce_log(struct mce *mce)
        mcelog.entry[entry].finished = 1;
        wmb();
 
-       if (!test_and_set_bit(0, &console_logged))
-               notify_user = 1;
+       set_bit(0, &notify_user);
 }
 
 static void print_mce(struct mce *m)

@@ -167,15 +169,19 @@ static inline void mce_get_rip(struct mc
        }
 }
 
-static void do_mce_trigger(void)
+/*
+ * This is only called in normal interrupt context.  This is where we do
+ * anything we need to alert userspace.  This is called directly from the
+ * poller and also from entry.S thanks to TIF_MCE_NOTIFY.
+ */
+void mce_notify_user(void)
 {
-       static atomic_t mce_logged;
-       int events = atomic_read(&mce_events);
-       if (events != atomic_read(&mce_logged) && trigger[0]) {
-               /* Small race window, but should be harmless.  */
-               atomic_set(&mce_logged, events);
+       clear_thread_flag(TIF_MCE_NOTIFY);
+       clear_bit(0, &notify_user);
+
+       wake_up_interruptible(&mce_wait);
+       if (trigger[0])
                call_usermodehelper(trigger, trigger_argv, NULL, -1);
-       }
 }
 
 /*

@@ -251,12 +257,8 @@ void do_machine_check(struct pt_regs * r
        }
 
        /* Never do anything final in the polling timer */
-       if (!regs) {
-               /* Normal interrupt context here. Call trigger for any new
-                  events. */
-               do_mce_trigger();
+       if (!regs)
                goto out;
-       }
 
        /* If we didn't find an uncorrectable error, pick
           the last one (shouldn't happen, just being safe). */

@@ -288,6 +290,9 @@ void do_machine_check(struct pt_regs * r
                        do_exit(SIGBUS);
        }
 
+       /* notify userspace ASAP */
+       set_thread_flag(TIF_MCE_NOTIFY);
+
  out:
        /* Last thing done in the machine check exception to clear state. */
        wrmsrl(MSR_IA32_MCG_STATUS, 0);

@@ -344,20 +349,18 @@ static void mcheck_timer(struct work_str
        on_each_cpu(mcheck_check_cpu, NULL, 1, 1);
 
        /*
-        * It's ok to read stale data here for notify_user and
-        * console_logged as we'll simply get the updated versions
-        * on the next mcheck_timer execution and atomic operations
-        * on console_logged act as synchronization for notify_user
-        * writes.
+        * It's ok to read stale data here for notify_user as we'll simply
+        * get the updated version on the next mcheck_timer execution.
         */
-       if (notify_user && console_logged) {
+       if (notify_user) {
                static unsigned long last_print;
                unsigned long now = jiffies;
 
+               /* alert userspace */
+               mce_notify_user();
+
                /* if we logged an MCE, reduce the polling interval */
                next_interval = max(next_interval/2, HZ/100);
-               notify_user = 0;
-               clear_bit(0, &console_logged);
                if (time_after_eq(now, last_print + (check_interval*HZ))) {
                        last_print = now;
                        printk(KERN_INFO "Machine check events logged\n");

@@ -530,6 +533,14 @@ static ssize_t mce_read(struct file *fil
        return err ? -EFAULT : buf - ubuf;
 }
 
+static unsigned int mce_poll(struct file *file, poll_table *wait)
+{
+       poll_wait(file, &mce_wait, wait);
+       if (rcu_dereference(mcelog.next))
+               return POLLIN | POLLRDNORM;
+       return 0;
+}
+
 static int mce_ioctl(struct inode *i, struct file *f,unsigned int cmd, unsigned long arg)
 {
        int __user *p = (int __user *)arg;

@@ -554,6 +565,7 @@ static int mce_ioctl(struct inode *i, st
 
 static const struct file_operations mce_chrdev_ops = {
        .read = mce_read,
+       .poll = mce_poll,
        .ioctl = mce_ioctl,
 };
 

diff -pruN linux-2.6.20+th/include/asm-x86_64/thread_info.h linux-2.6.20+th2v2/include/asm-x86_64/thread_info.h

include/asm-x86_64/thread_info.h 2007-04-27 23:13:59.000000000 -0700 renamed from include/asm-x86_64/thread_info.h 2007-02-04 10:44:54.000000000 -0800 +2 -0

@@ -115,6 +115,7 @@ static inline struct thread_info *stack_
 #define TIF_SYSCALL_AUDIT      7       /* syscall auditing active */
 #define TIF_SECCOMP            8       /* secure computing */
 #define TIF_RESTORE_SIGMASK    9       /* restore signal mask in do_signal */
+#define TIF_MCE_NOTIFY         10      /* notify userspace of an MCE */
 /* 16 free */
 #define TIF_IA32               17      /* 32bit process */
 #define TIF_FORK               18      /* ret_from_fork */

@@ -133,6 +134,7 @@ static inline struct thread_info *stack_
 #define _TIF_SYSCALL_AUDIT     (1<<TIF_SYSCALL_AUDIT)
 #define _TIF_SECCOMP           (1<<TIF_SECCOMP)
 #define _TIF_RESTORE_SIGMASK   (1<<TIF_RESTORE_SIGMASK)
+#define _TIF_MCE_NOTIFY                (1<<TIF_MCE_NOTIFY)
 #define _TIF_IA32              (1<<TIF_IA32)
 #define _TIF_FORK              (1<<TIF_FORK)
 #define _TIF_ABI_PENDING       (1<<TIF_ABI_PENDING)