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

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Message-ID: <BANLkTikxcfGYAmKf5QEAwJjDLdo6_k6zaw@mail.gmail.com>
Date:	Thu, 12 May 2011 18:43:05 +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 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?

Why is %ptc better than

char comm[...];
get_task_comm(comm, current);
printk("%s: ....", comm);

?
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