linux-kernel - Re: perf: 3.17 another perf

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Message-ID: <alpine.DEB.2.11.1410171015540.15673@vincent-weaver-1.umelst.maine.edu>
Date:	Fri, 17 Oct 2014 10:19:50 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Vince Weaver <vincent.weaver@...ne.edu>
cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: 3.17 another perf_fuzzer lockup

On Thu, 16 Oct 2014, Vince Weaver wrote:

> > It looks like the
> > 			else if (task->perf_event_ctxp[ctxn])
> > 	                        err = -EAGAIN;
> 
> It is indeed stuck there, waiting for task->perf_event_ctxp[1] to get
> set to zero, which never happens.
> 

OK, so with some more printk()s, it looks like somehow the parent thread
is trying to open a software event on itself.

	task->perf_event_ctxp[1]

has a valid pointer, but the ctx it points to has a ctx->lock of 0.

So perf_lock_task_context() always returns NULL.

So in find_get_context()
	we get stuck in an infinite retry loop, waiting forever for
	either ctx->lock to go positive or for task->perf_event_ctxp[1]
	to go NULL, neither of which is going to happen.

Now to find out why this could happen.  Probably something to do with 
crazy RCU magic :(

Vince

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