linux-kernel - Re: [GIT PULL rcu/next] RCU commits for 3.1

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Message-ID: <1320686777.17809.34.camel@twins>
Date:	Mon, 07 Nov 2011 18:26:17 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	paulmck@...ux.vnet.ibm.com, Li Zefan <lizf@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, eric.dumazet@...il.com,
	shaohua.li@...el.com, ak@...ux.intel.com, mhocko@...e.cz,
	alex.shi@...el.com, efault@....de, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>
Subject: Re: [GIT PULL rcu/next] RCU commits for 3.1

On Mon, 2011-11-07 at 17:12 +0000, Stephane Eranian wrote:
> I think on that path:
> 
> >>> [<8108aa02>] perf_event_enable_on_exec+0x1d2/0x1e0
> >>> [<81063764>] ? __lock_release+0x54/0xb0
> >>> [<8108cca8>] perf_event_comm+0x18/0x60
> >>> [<810d1abd>] ? set_task_comm+0x5d/0x80
> >>> [<81af622d>] ? _raw_spin_unlock+0x1d/0x40
> >>> [<810d1ac4>] set_task_comm+0x64/0x80
> 
> We are neither holding the rcu_read_lock() nor the task_lock() but we
> are operating on the current task. The task cannot just vanish. So
> the rcu_dereference() and lock_is_held() macros may detect a false
> positive in that case. Yet, I doubt this would be the only place.... 

Well, normally being current doesn't guarantee your cgroup won't
disappear. The perf stuff hwoever takes refs and is synced against
->attach() by virtue of it calling perf_cgroup_switch() etc..
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