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Subject: Re: [PATCH 2/2] x86: Make the x86-64 stacktrace code safely
 callable from scheduler
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>,
        LKML <linux-kernel@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>
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Date: Fri, 13 May 2011 15:19:58 +0200
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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.

>                                         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.