linux-kernel - Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect task->comm access

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Message-ID: <20110517212734.GB28054@elte.hu>
Date:	Tue, 17 May 2011 23:27:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Joe Perches <joe@...ches.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Andy Whitcroft <apw@...onical.com>,
	Jiri Slaby <jirislaby@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect
 task->comm access


* John Stultz <john.stultz@...aro.org> wrote:

> The implicit rules for current->comm access being safe without locking are no 
> longer true. Accessing current->comm without holding the task lock may result 
> in null or incomplete strings (however, access won't run off the end of the 
> string).

This is rather unfortunate - task->comm is used in a number of performance 
critical codepaths such as tracing.

Why does this matter so much? A NULL string is not a big deal.

Note, since task->comm is 16 bytes there's the CMPXCHG16B instruction on x86 
which could be used to update it atomically, should atomicity really be 
desired.

Thanks,

	Ingo
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