FAILED: patch "[PATCH] rcu: Allow for page faults in NMI handlers" failed to apply to 4.9-stable tree

From 28585a832602747cbfa88ad8934013177a3aae38 Mon Sep 17 00:00:00 2001

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

Quoted 1 line

From 28585a832602747cbfa88ad8934013177a3aae38 Mon Sep 17 00:00:00 2001

From: "Paul E. McKenney" <[email protected]>
Date: Fri, 22 Sep 2017 14:10:22 -0700
Subject: [PATCH] rcu: Allow for page faults in NMI handlers

A number of architecture invoke rcu_irq_enter() on exception entry in order to allow RCU read-side critical sections in the exception handler when the exception is from an idle or nohz_full CPU. This works, at least unless the exception happens in an NMI handler. In that case, rcu_nmi_enter() would already have exited the extended quiescent state, which would mean that rcu_irq_enter() would (incorrectly) cause RCU to think that it is again in an extended quiescent state. This will in turn result in lockdep splats in response to later RCU read-side critical sections.

This commit therefore causes rcu_irq_enter() and rcu_irq_exit() to take no action if there is an rcu_nmi_enter() in effect, thus avoiding the unscheduled return to RCU quiescent state. This in turn should make the kernel safe for on-demand RCU voyeurism.

Link http://lkml.kernel.org/r/[email protected] open here

Cc [email protected]

Fixes 0be964be0 ("module: Sanitize RCU usage and locking")

Reported-by Steven Rostedt <[email protected]>

Signed-off-by Paul E. McKenney <[email protected]>

Signed-off-by Steven Rostedt (VMware) <[email protected]>

kernel/rcu/tree.c +10 -0

index 51d4c3acf32d..63bee8e1b193 100644
@@ -888,6 +888,11 @@ void rcu_irq_exit(void)
 
 	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");
 	rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	/* Page faults can happen in NMI handlers, so check... */
+	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+		return;
+
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
 		     rdtp->dynticks_nesting < 1);
 	if (rdtp->dynticks_nesting <= 1) {

@@ -1020,6 +1025,11 @@ void rcu_irq_enter(void)
 
 	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!");
 	rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	/* Page faults can happen in NMI handlers, so check... */
+	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+		return;
+
 	oldval = rdtp->dynticks_nesting;
 	rdtp->dynticks_nesting++;
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&

Paul,

Can you backport this patch to 4.9 and 4.4 if it is required. The other patches I posted (and also failed to apply, but should be backported) depend on this patch, if it is an issue with those kernels.

-- Steve

Quoted 67 lines · On Mon, 02 Oct 2017 13:44:37 +0200 <[email protected]> wrote:

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

Quoted 1 line

From 28585a832602747cbfa88ad8934013177a3aae38 Mon Sep 17 00:00:00 2001

From: "Paul E. McKenney" <[email protected]>
Date: Fri, 22 Sep 2017 14:10:22 -0700
Subject: [PATCH] rcu: Allow for page faults in NMI handlers

A number of architecture invoke rcu_irq_enter() on exception entry in order to allow RCU read-side critical sections in the exception handler when the exception is from an idle or nohz_full CPU. This works, at least unless the exception happens in an NMI handler. In that case, rcu_nmi_enter() would already have exited the extended quiescent state, which would mean that rcu_irq_enter() would (incorrectly) cause RCU to think that it is again in an extended quiescent state. This will in turn result in lockdep splats in response to later RCU read-side critical sections.

This commit therefore causes rcu_irq_enter() and rcu_irq_exit() to take no action if there is an rcu_nmi_enter() in effect, thus avoiding the unscheduled return to RCU quiescent state. This in turn should make the kernel safe for on-demand RCU voyeurism.

Link http://lkml.kernel.org/r/[email protected] open here

Cc [email protected]

Fixes 0be964be0 ("module: Sanitize RCU usage and locking")

Reported-by Steven Rostedt <[email protected]>

Signed-off-by Paul E. McKenney <[email protected]>

Signed-off-by Steven Rostedt (VMware) <[email protected]>

kernel/rcu/tree.c +10 -0

index 51d4c3acf32d..63bee8e1b193 100644
@@ -888,6 +888,11 @@ void rcu_irq_exit(void)
 
 	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");
 	rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	/* Page faults can happen in NMI handlers, so check... */
+	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+		return;
+
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
 		     rdtp->dynticks_nesting < 1);
 	if (rdtp->dynticks_nesting <= 1) {

@@ -1020,6 +1025,11 @@ void rcu_irq_enter(void)
 
 	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!");
 	rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	/* Page faults can happen in NMI handlers, so check... */
+	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+		return;
+
 	oldval = rdtp->dynticks_nesting;
 	rdtp->dynticks_nesting++;
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&

Quoted 6 lines · Steven Rostedt

Paul,

Can you backport this patch to 4.9 and 4.4 if it is required. The other patches I posted (and also failed to apply, but should be backported) depend on this patch, if it is an issue with those kernels.

No problem!

How do we determine whether or not it is required?

Do we need to ask one of Bruce or Nick? ;-)

Quoted 11 lines · On Mon, 2 Oct 2017 12:38:57 -0700 "Paul E. McKenney"…

Quoted 6 lines · Steven Rostedt

Paul,

Can you backport this patch to 4.9 and 4.4 if it is required. The other patches I posted (and also failed to apply, but should be backported) depend on this patch, if it is an issue with those kernels.

No problem!

How do we determine whether or not it is required?

Can rcu_irq_enter() be called within a rcu_nmi_enter() in these kernels? If not, that needs to be fixed. Because nmi's can still page fault.

Quoted 3 lines

Do we need to ask one of Bruce or Nick? ;-)

The bug I fixed is actually there, but lockdep doesn't complain. The module code that uses synchronize_sched() which requires RCU to be watching when preemption is disabled. RCU isn't watching is some of these cases that are called. Recent kernels added rcu annotations into these preempt disabled paths which caused lockdep to report the issue. The older kernels that had the bug just didn't have lockdep complain about it.