Subject: Re: Random scheduler/unaligned accesses crashes with perf lock
 events on sparc 64
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: David Miller <davem@davemloft.net>, sparclinux@vger.kernel.org,
       linux-kernel@vger.kernel.org, mingo@elte.hu, acme@redhat.com,
       paulus@samba.org, Mike Galbraith <efault@gmx.de>
In-Reply-To: <20100406113830.GF5147@nowhere>
References: <20100405065701.GC5127@nowhere>
	 <20100405.122233.188421941.davem@davemloft.net>
	 <20100405194055.GA5265@nowhere>
	 <20100406.025049.267615796.davem@davemloft.net>
	 <20100406113830.GF5147@nowhere>
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Date: Tue, 06 Apr 2010 13:51:56 +0200
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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@gmail.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

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