linux-kernel - Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

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Date:	Thu, 12 May 2011 18:45:05 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, "Ted Ts'o" <tytso@....edu>,
	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
Subject: Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

On Thu, May 12, 2011 at 6:43 PM, Américo Wang <xiyou.wangcong@...il.com> wrote:
> On Thu, May 12, 2011 at 5:02 AM, John Stultz <john.stultz@...aro.org> wrote:
>> 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@...aro.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.
>>
>
> Sorry, I meant why not adding the locking into a wrapper function,
> probably get_task_comm() and let the users to call it directly?
>

Ahhh, never mind, I see the points now... Then it is fine. :)
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