linux-kernel - Re: Random scheduler/unaligned accesses crashes with perf lock events on sparc 64

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Date:	Tue, 06 Apr 2010 14:54:10 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	David Miller <davem@...emloft.net>, sparclinux@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, acme@...hat.com,
	paulus@...ba.org
Subject: Re: Random scheduler/unaligned accesses crashes with perf lock
 events on sparc 64

On Tue, 2010-04-06 at 13:51 +0200, Peter Zijlstra wrote:
> On Tue, 2010-04-06 at 13:38 +0200, Frederic Weisbecker wrote:
> > On Tue, Apr 06, 2010 at 02:50:49AM -0700, David Miller wrote:
> > > From: Frederic Weisbecker <fweisbec@...il.com>
> > > Date: Mon, 5 Apr 2010 21:40:58 +0200
> > > 
> > > > It happens without CONFIG_FUNCTION_TRACER as well (but it happens
> > > > when the function tracer runs). And I hadn't your
> > > > perf_arch_save_caller_regs() when I triggered this.
> > > 
> > > I figured out the problem, it's NMIs.  As soon as I disable all of the
> > > NMI watchdog code, the problem goes away.
> > > 
> > > This is because some parts of the NMI interrupt handling path are not
> > > marked with "notrace" and the various tracer code paths use
> > > local_irq_disable() (either directly or indirectly) which doesn't work
> > > with sparc64's NMI scheme.  These essentially turn NMIs back on in the
> > > NMI handler before the NMI condition has been cleared, and thus we can
> > > re-enter with another NMI interrupt.
> > > 
> > > We went through this for perf events, and we just made sure that
> > > local_irq_{enable,disable}() never occurs in any of the code paths in
> > > perf events that can be reached via the NMI interrupt handler.  (the
> > > only one we had was sched_clock() and that was easily fixed)
> > 
> > 
> > 
> > That reminds me we have a new pair of local_irq_disable/enable
> > in perf_event_task_output(), which path can be taken by hardware
> > pmu events.
> > 
> > See this patch:
> > 
> > 8bb39f9aa068262732fe44b965d7a6eb5a5a7d67
> > perf: Fix 'perf sched record' deadlock
> 
> ARGH.. yes
> 
> Also, I guess that should live in perf_output_lock/unlock() not in
> perf_event_task_output().
> 
> Egads, how to fix that

Damn, so deadlock fix isn't a fix.  No idea.

	-Mike

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