linux-kernel - Re: perf: fuzzer getting stuck with slow memory leak

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Date:	Tue, 27 May 2014 17:00:07 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Vince Weaver <vincent.weaver@...ne.edu>
cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: perf: fuzzer getting stuck with slow memory leak

On Tue, 27 May 2014, Vince Weaver wrote:

> On Tue, 27 May 2014, Vince Weaver wrote:
> 
> > OK, the problem is in:
> > 
> > 	kernel/events/core.c find_get_context()
> > 
> > retry:
> > 	perf_lock_task_context() returns NULL.
> > 	alloc_perf_context()
> > 	task->perf_event_ctxp[ctxn] is *not* NULL
> > 	err == EAGAIN so jump to retry
> > 

so after sticking some trace_printk()s in the code, it turns out
that for the software PMU

perf_lock_task_context() returns NULL beause 
	!atomic_inc_not_zero(&ctx->refcount)
is zero, but
	task->perf_event_ctxp[ctxn] still has a value.

And since those conditions never change it just loops forever, stuck.

I'm not sure how the reference count is getting messed up.
This is reproducible so if there's anything useful I can turn on
tracing-wise let me know.

Vince
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