linux-kernel - Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

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Date:   Mon, 29 Jul 2019 11:21:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Eiichi Tsukata <devel@...ukata.com>
Cc:     joel@...lfernandes.org, paulmck@...ux.vnet.ibm.com,
        tglx@...utronix.de, peterz@...radead.org, mingo@...hat.com,
        fweisbec@...il.com, luto@...capital.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

On Mon, 29 Jul 2019 10:07:34 +0900
Eiichi Tsukata <devel@...ukata.com> wrote:

> If context tracking is enabled, causing page fault in preemptirq
> irq_enable or irq_disable events triggers the following RCU EQS warning.
> 
> Reproducer:
> 
>   // CONFIG_PREEMPTIRQ_EVENTS=y
>   // CONFIG_CONTEXT_TRACKING=y
>   // CONFIG_RCU_EQS_DEBUG=y
>   # echo 1 > events/preemptirq/irq_disable/enable
>   # echo 1 > options/userstacktrace

So the problem is only with userstacktrace enabled?


>  }
> diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
> index 4d8e99fdbbbe..031b51cb94d0 100644
> --- a/kernel/trace/trace_preemptirq.c
> +++ b/kernel/trace/trace_preemptirq.c
> @@ -10,6 +10,7 @@
>  #include <linux/module.h>
>  #include <linux/ftrace.h>
>  #include <linux/kprobes.h>
> +#include <linux/context_tracking.h>
>  #include "trace.h"
>  
>  #define CREATE_TRACE_POINTS
> @@ -49,9 +50,14 @@ NOKPROBE_SYMBOL(trace_hardirqs_off);
>  
>  __visible void trace_hardirqs_on_caller(unsigned long caller_addr)
>  {
> +	enum ctx_state prev_state;
> +
>  	if (this_cpu_read(tracing_irq_cpu)) {
> -		if (!in_nmi())
> +		if (!in_nmi()) {

This is a very high fast path (for tracing irqs off and such). Instead
of adding a check here for a case that is seldom used (userstacktrace
and tracing irqs on/off). Move this to surround the userstack trace
code.

-- Steve


> +			prev_state = exception_enter();
>  			trace_irq_enable_rcuidle(CALLER_ADDR0, caller_addr);
> +			exception_exit(prev_state);
> +		}
>  		tracer_hardirqs_on(CALLER_ADDR0, caller_addr);
>  		this_cpu_write(tracing_irq_cpu, 0);
>  	}
> @@ -63,11 +69,16 @@ NOKPROBE_SYMBOL(trace_hardirqs_on_caller);
>  
>  __visible void trace_hardirqs_off_caller(unsigned long caller_addr)
>  {
> +	enum ctx_state prev_state;
> +
>  	if (!this_cpu_read(tracing_irq_cpu)) {
>  		this_cpu_write(tracing_irq_cpu, 1);
>  		tracer_hardirqs_off(CALLER_ADDR0, caller_addr);
> -		if (!in_nmi())
> +		if (!in_nmi()) {
> +			prev_state = exception_enter();
>  			trace_irq_disable_rcuidle(CALLER_ADDR0, caller_addr);
> +			exception_exit(prev_state);
> +		}
>  	}
>  
>  	lockdep_hardirqs_off(CALLER_ADDR0);