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

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Date:	Tue, 17 May 2011 15:27:05 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Ingo Molnar <mingo@...e.hu>
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

On Tue, 2011-05-17 at 23:27 +0200, Ingo Molnar wrote:
> * 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.

I'll defer to KOSAKI Motohiro and David on this bit. :)

> 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.

Could we use this where cmpxchg16b is available and fall back to locking
if not? Or does that put too much of a penalty on arches that don't have
cmpxchg16b support?

Alternatively, we can have locked accessors that are safe in the
majority of slow-path warning printks, and provide unlocked accessors
for cases where the performance is critical and the code can properly
handle possibly incomplete comms.

thanks
-john



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