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

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Message-ID: <20110512215544.GC20939@elte.hu>
Date:	Thu, 12 May 2011 23:55:44 +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:

> On Thu, May 12, 2011 at 11:28:10PM +0200, Ingo Molnar wrote:
> > 
> > * 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().
> 
> Good point, but then lockdep itself might trigger a stacktrace from local_irq_save,
> leading to a stacktrace recursion.
> 
> I can use raw_local_irq_disable(), or may be have a stacktrace recursion 
> protection. I fear the second solution could lead us to potentially lose 
> useful information if a stacktrace interrupts another one. Ok these are 
> extreme cases...

i think raw_local_irq_disable() would be justified in this case.

Thanks,

	Ingo
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