This is a note to let you know that I've just added the patch titled rcu: Allow for page faults in NMI handlers to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: rcu-allow-for-page-faults-in-nmi-handlers.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 28585a832602747cbfa88ad8934013177a3aae38 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> Date: Fri, 22 Sep 2017 14:10:22 -0700 Subject: rcu: Allow for page faults in NMI handlers From: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> commit 28585a832602747cbfa88ad8934013177a3aae38 upstream. 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/20170922211022.GA18084@xxxxxxxxxxxxxxxxxx Fixes: 0be964be0 ("module: Sanitize RCU usage and locking") Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/rcu/tree.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -792,8 +792,13 @@ void rcu_irq_exit(void) long long oldval; struct rcu_dynticks *rdtp; - 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; + + RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!"); oldval = rdtp->dynticks_nesting; rdtp->dynticks_nesting--; WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && @@ -930,8 +935,13 @@ void rcu_irq_enter(void) struct rcu_dynticks *rdtp; long long oldval; - 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; + + RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!"); oldval = rdtp->dynticks_nesting; rdtp->dynticks_nesting++; WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && Patches currently in stable-queue which might be from paulmck@xxxxxxxxxxxxxxxxxx are queue-4.9/rcu-allow-for-page-faults-in-nmi-handlers.patch