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

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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


On Mon, 02 Oct 2017 13:44:37 +0200
<gregkh@xxxxxxxxxxxxxxxxxxx> 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 <stable@xxxxxxxxxxxxxxx>.
> 
> thanks,
> 
> greg k-h
> 
> ------------------ original commit in Linus's tree ------------------
> 
> >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: [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/20170922211022.GA18084@xxxxxxxxxxxxxxxxxx
> 
> Cc: stable@xxxxxxxxxxxxxxx
> 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>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 51d4c3acf32d..63bee8e1b193 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -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) &&




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