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Subject: Re: [BUG] perf: hard lockup when using perf-sched
From: Mike Galbraith <efault@gmx.de>
To: Li Zefan <lizf@cn.fujitsu.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <peterz@infradead.org>,
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       Paul Mackerras <paulus@samba.org>
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Date: Thu, 25 Mar 2010 10:27:21 +0100
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On Thu, 2010-03-25 at 16:04 +0800, Li Zefan wrote:
> Mike Galbraith wrote:
> > On Wed, 2010-03-24 at 08:32 +0100, Mike Galbraith wrote:
> > 
> >> I just saw this, hunted down your testcase and tried it here.  Looks
> >> like perf_output_lock() wedged box.
> > 
> > (turns on frame pointers, and adds noinline)
> > 
> 
> Thanks! Then who's going to fix this...

Well, that kinda depends on whether I figure out how the heck it's all
supposed to work before somebody else whacks it or not.

ATM, I've instrumented, know _what's_ happening, but find myself saying
"wtf?" a lot, especially wrt handle->locked.  The act of attempting to
lock a handle declares it unlocked, turning perf_output_unlock() into a
noop, which looks a bit strange.  We're spinning on those "unlocked"
locks, all left genuinely locked by one CPU.  I just whacked the thing,
and am very likely about to see in yet another trace.

Locking is hard, "curious construct" locking is even harder :)

	-Mike