linux-kernel - Re: [PATCH 3/3] comm: ext4: Protect task->comm access by using get_task

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Date:	Wed, 4 May 2011 16:36:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org
Subject: Re: [PATCH 3/3] comm: ext4: Protect task->comm access by using
 get_task_comm()

On Thu, 28 Apr 2011 14:35:32 -0700 (PDT)
David Rientjes <rientjes@...gle.com> wrote:

> On Wed, 27 Apr 2011, John Stultz wrote:
> 
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index 7b80d54..d37414e 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -124,11 +124,15 @@ ext4_file_write(struct kiocb *iocb, const struct iovec *iov,
> >  		static unsigned long unaligned_warn_time;
> >  
> >  		/* Warn about this once per day */
> > -		if (printk_timed_ratelimit(&unaligned_warn_time, 60*60*24*HZ))
> > +		if (printk_timed_ratelimit(&unaligned_warn_time, 60*60*24*HZ)) {
> > +			char comm[TASK_COMM_LEN];
> > +
> > +			get_task_comm(comm, current);
> >  			ext4_msg(inode->i_sb, KERN_WARNING,
> >  				 "Unaligned AIO/DIO on inode %ld by %s; "
> >  				 "performance will be poor.",
> > -				 inode->i_ino, current->comm);
> > +				 inode->i_ino, comm);
> > +		}
> >  		mutex_lock(ext4_aio_mutex(inode));
> >  		ext4_aiodio_wait(inode);
> >  	}
> 
> Thanks very much for looking into concurrent readers of current->comm, 
> John!
> 
> This patch in the series demonstrates one of the problems with using 
> get_task_comm(), however: we must allocate a 16-byte buffer on the stack 
> and that could become risky if we don't know its current depth.  We may be 
> particularly deep in the stack and then cause an overflow because of the 
> 16 bytes.
> 
> I'm wondering if it would be better for ->comm to be protected by a 
> spinlock (or rwlock) other than ->alloc_lock and then just require readers 
> to take the lock prior to dereferencing it?  That's what is done in the 
> oom killer with task_lock().  Perhaps you could introduce new 
> task_comm_lock() and task_comm_unlock() to prevent the extra stack usage 
> in over 300 locations within the kernel?

16 bytes isn't all that much.  It's just two longs worth.

I'm suspecting that approximately 100% of the get_task_comm() callsites
are using it for a printk, so how about we add a %p thingy for it then
zap lots of code?

I read the changelogs and can't work out why a seqlock was added.  What
was wrong with the task_lock()?

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