Subject: Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm
From: John Stultz <john.stultz@linaro.org>
To: =?ISO-8859-1?Q?Am=E9rico?= Wang <xiyou.wangcong@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, "Ted Ts'o" <tytso@mit.edu>,
        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
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	 <BANLkTikXyqddLbQKyDYFrAwq9DamDj--AQ@mail.gmail.com>
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Date: Wed, 11 May 2011 14:02:56 -0700
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On Wed, 2011-05-11 at 17:33 +0800, Américo Wang wrote:
> On Wed, May 11, 2011 at 8:23 AM, John Stultz <john.stultz@linaro.org> wrote:
> > Acessing task->comm requires proper locking. However in the past
> > access to current->comm could be done without locking. This
> > is no longer the case, so all comm access needs to be done
> > while holding the comm_lock.
> >
> > In my attempt to clean up unprotected comm access, I've noticed
> > most comm access is done for printk output. To simpify correct
> > locking in these cases, I've introduced a new %ptc format,
> > which will safely print the corresponding task's comm.
> >
> > Example use:
> > printk("%ptc: unaligned epc - sending SIGBUS.\n", current);
> >
> 
> Why do you hide current->comm behide printk?
> How is this better than printk("%s: ....", task_comm(current)) ?

So to properly access current->comm, you need to hold the task-lock (or
with my new patch set, the comm_lock). Rather then adding locking to all
the call sites that printk("%s ...", current->comm), I'm suggesting we
add a new %ptc method which will handle the locking for you.

thanks
-john


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