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Subject: Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     joel@joelfernandes.org, paulmck@linux.vnet.ibm.com,
        tglx@linutronix.de, peterz@infradead.org, mingo@redhat.com,
        fweisbec@gmail.com, luto@amacapital.net,
        linux-kernel@vger.kernel.org
References: <20190729010734.3352-1-devel@etsukata.com>
 <20190729112126.6554b141@gandalf.local.home>
From:   Eiichi Tsukata <devel@etsukata.com>
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Message-ID: <2ceec933-503e-5d58-60b4-85b491b017d4@etsukata.com>
Date:   Tue, 30 Jul 2019 11:00:42 +0900
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Thanks for comments.

On 2019/07/30 0:21, Steven Rostedt wrote:
> On Mon, 29 Jul 2019 10:07:34 +0900
> Eiichi Tsukata <devel@etsukata.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?

It can happen when tracing code causes page fault in preemptirq events.
For example, the following perf command also hit the warning:

  # perf record -e 'preemptirq:irq_enable' -g ls


>>  
>>  __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

If the problem was only with userstacktrace, it will be reasonable to
surround only the userstack unwinder. But the situation is similar to
the previous "tracing vs CR2" case. As Peter taught me in
https://lore.kernel.org/lkml/20190708074823.GV3402@hirez.programming.kicks-ass.net/
there are some other codes likely to to user access.
So I surround preemptirq events earlier.