linux-kernel - Re: [PATCH] perf: Fix RCU dereference check in perf_event

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Date:	Thu, 22 Mar 2012 10:53:16 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ari Savolainen <ari.m.savolainen@...il.com>
Cc:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Fix RCU dereference check in perf_event_comm

On Thu, 2012-03-22 at 01:43 +0200, Ari Savolainen wrote:
> The warning below is printed when executing a command like
> sudo perf record su - user -c "echo hello"
> 
> It's fixed by moving the call of perf_event_comm to be protected
> by the task lock.

That seems like a rather poor solution since it increases the lock hold
time for no explained reason.

> include/linux/cgroup.h:567 suspicious rcu_dereference_check() usage!

>  [<ffffffff8109be55>] lockdep_rcu_suspicious+0xe5/0x100
>  [<ffffffff811131fa>] perf_event_comm+0x37a/0x4d0

So where exactly is this, perf_event_comm_event() takes rcu_read_lock()
so I presume its before that.
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