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Date: Tue, 6 Apr 2010 13:38:34 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: David Miller <davem@davemloft.net>
Cc: sparclinux@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu,
       acme@redhat.com, a.p.zijlstra@chello.nl, paulus@samba.org
Subject: Re: Random scheduler/unaligned accesses crashes with perf lock
	events on sparc 64
Message-ID: <20100406113830.GF5147@nowhere>
References: <20100405065701.GC5127@nowhere> <20100405.122233.188421941.davem@davemloft.net> <20100405194055.GA5265@nowhere> <20100406.025049.267615796.davem@davemloft.net>
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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

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