linux-kernel - Re: [PATCH 2/2] x86: Make the x86-64 stacktrace code safely callable from scheduler

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Message-ID: <20110512212810.GA17596@elte.hu>
Date:	Thu, 12 May 2011 23:28:10 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/2] x86: Make the x86-64 stacktrace code safely callable
 from scheduler


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> Avoid potential scheduler recursion and deadlock from the
> stacktrace code by avoiding rescheduling when we re-enable
> preemption.
> 
> This robustifies some scheduler trace events like sched switch
> when they are used to produce callchains in perf or ftrace.

> -	put_cpu();
> +
> +	/* We want stacktrace to be computable anywhere, even in the scheduler */
> +	preempt_enable_no_resched();

So what happens if a callchain profiling happens to be interrupted by a hardirq 
and the interrupt reschedules the current task? We'll miss the reschedule, 
right?

preempt_enable_no_resched() is not a magic 'solve scheduler recursions' bullet 
- it's to be used only if something else will guarantee the preemption check! 
But nothing guarantees it here AFAICS.

A better fix would be to use local_irq_save()/restore().

Thanks,

	Ingo
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