linux-kernel - Re: [perf] more perf_fuzzer memory corruption

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Message-ID: <alpine.DEB.2.10.1405011443410.19874@vincent-weaver-1.umelst.maine.edu>
Date:	Thu, 1 May 2014 14:49:01 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Vince Weaver <vincent.weaver@...ne.edu>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [perf] more perf_fuzzer memory corruption


OK, humor me a bit here.

I'm looking at the buggy trace and comparing against a "good" trace where 
the bug doesn't happen.

It is a rance condition of sorts, because it's just a 10us or so 
interleaving of calls that causes the bug to happen or not.

In the good trace:

	[parent] __perf_event_task_sched_out (and hence perf_swevent_del)
	[child]  perf_release

In the buggy trace:

	[child] perf_release
	[parent] __perf_event_task_sched_out (perf_swevent_del never happens)


perf_swevent_del calls
	hlist_del_rcu(event->hlist_entry)
to remove the event from the swevent hlist.

Now in theory perf_release() calls sw_perf_event_destroy() which you
would think would also call the above.  Instead it does
	 swevent_hlist_put_cpu(event, cpu);
which does all kinds of weird hash stuff that I don't follow.

Should the above two be equivelent?  Is it reference counting in there 
with if (!--swhash->hlist_refcount) causing the issue?

Anyway I'm tired of staring at traces for the moment.

Vince
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