Date: Tue, 17 May 2011 23:27:34 +0200
From: Ingo Molnar <mingo@elte.hu>
To: John Stultz <john.stultz@linaro.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Joe Perches <joe@perches.com>,
        Michal Nazarewicz <mina86@mina86.com>,
        Andy Whitcroft <apw@canonical.com>, Jiri Slaby <jirislaby@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        David Rientjes <rientjes@google.com>,
        Dave Hansen <dave@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 1/3] comm: Introduce comm_lock spinlock to protect
 task->comm access
Message-ID: <20110517212734.GB28054@elte.hu>
References: <1305665263-20933-1-git-send-email-john.stultz@linaro.org>
 <1305665263-20933-2-git-send-email-john.stultz@linaro.org>
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* John Stultz <john.stultz@linaro.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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