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

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Date:	Fri, 13 May 2011 15:29:19 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] x86: Make the x86-64 stacktrace code safely
 callable from scheduler

On Fri, May 13, 2011 at 03:19:58PM +0200, Peter Zijlstra wrote:
> On Fri, 2011-05-13 at 14:48 +0200, Frederic Weisbecker wrote:
> > I haven't observed any deadlock. trace events disable preemption and
> > other tracers do too (my changelog was buggy).
> > 
> > I just worried about potential other users, like a WARN_ON in the
> > scheduler or so.
> > 
> > My worry is the following scenario:
> > 
> > schedule() {
> >         acquire(rq)
> >                                                         set_tsk_need_resched
> >         WARN_ON() {
> >                 stack_trace() {
> >                         preempt_enable() {
> >                                 preempt_schedule() {
> 
> Would never happen, because rq->lock is a spinlock which holds another
> preempt count so preempt_enable() would never schedule.

Oh right.

> 
> >                                         acquire(rq)
> >                                 }
> >                         }
> >                 }
> >         }
> > }
> 
> 
> 
> > I don't know if it happens that one set TIF_NEED_RESCHED remotely,
> 
> Yes
> 
> > or if TIF_NEED_RESCHED can be set when we hold the rq,
> 
> Yes
> 
> >  and then we
> > can be followed by a WARN_ON, ...
> 
> Not quite sure, but possible.
> 
> > So I preferred to be careful. 
> 
> Still not quite seeing how all things could go bang.

Nah, forget about that, I was just confused ;)
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