linux-kernel - Re: [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access

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Message-ID: <1305311276.2680.34.camel@work-vm>
Date:	Fri, 13 May 2011 11:27:56 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, "Ted Ts'o" <tytso@....edu>,
	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 1/3] comm: Introduce comm_lock seqlock to protect
 task->comm access

On Fri, 2011-05-13 at 20:13 +0900, KOSAKI Motohiro wrote:
> Hi
> 
> Sorry for the long delay.
> 
> >   char *get_task_comm(char *buf, struct task_struct *tsk)
> >   {
> > -	/* buf must be at least sizeof(tsk->comm) in size */
> > -	task_lock(tsk);
> > -	strncpy(buf, tsk->comm, sizeof(tsk->comm));
> > -	task_unlock(tsk);
> > +	unsigned long seq;
> > +
> > +	do {
> > +		seq = read_seqbegin(&tsk->comm_lock);
> > +
> > +		strncpy(buf, tsk->comm, sizeof(tsk->comm));
> > +
> > +	} while (read_seqretry(&tsk->comm_lock, seq));
> > +
> >   	return buf;
> >   }
> 
> Can you please explain why we should use seqlock? That said,
> we didn't use seqlock for /proc items. because, plenty seqlock
> write may makes readers busy wait. Then, if we don't have another
> protection, we give the local DoS attack way to attackers.

So you're saying that heavy write contention can cause reader
starvation? 

> task->comm is used for very fundamentally. then, I doubt we can
> assume write is enough rare. Why can't we use normal spinlock?

I think writes are likely to be fairly rare. Tasks can only name
themselves or sibling threads, so I'm not sure I see the risk here.

Mind going into more detail?

thanks
-john


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