linux-kernel - Re: [PATCH 0/4] v6 Improve task->comm locking situation

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Date:	Wed, 18 May 2011 09:58:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	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>, linux-mm@...ck.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/4] v6 Improve task->comm locking situation


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 18 May 2011 08:25:54 +0200 Ingo Molnar <mingo@...e.hu> wrote:
> 
> >   " Hey, this looks a bit racy and 'top' very rarely, on rare workloads that 
> >     play with ->comm[], might display a weird reading task name for a second, 
> >     amongst the many other temporarily nonsensical statistical things it 
> >     already prints every now and then. "
> 
> Well we should at least make sure that `top' won't run off the end of comm[] 
> and go oops.  I think that's guaranteed by the fact(s) that init_tasks's 
> comm[15] is zero and is always copied-by-value across fork and can never be 
> overwritten in any task_struct.

Correct.

> But I didn't check that.

I actually have a highly threaded app that uses PR_SET_NAME heavily and would 
have noticed any oopsing potential long ago.

Since ->comm is often observed from other tasks, regardless whether it's set 
from the prctl() or from the newfangled /proc vector, the race for seeing 
partial updates to ->comm always existed - for more than 10 years.

So the premise of the whole series is wrong: temporarily incomplete ->comm[]s 
were *always* possible and did not start 1.5+ years ago with:

  4614a696bd1c: procfs: allow threads to rename siblings via /proc/pid/tasks/tid/comm

when i see series being built on a fundamentally wrong premise i get a bit sad!

Thanks,

	Ingo
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