netdev - Re: Kernel crash after using new Intel NIC (igb)

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Message-ID: <1306305331.3305.22.camel@edumazet-laptop>
Date:	Wed, 25 May 2011 08:35:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Arun Sharma <asharma@...com>
Cc:	Maximilian Engelhardt <maxi@...monizer.de>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	StuStaNet Vorstand <vorstand@...sta.mhn.de>
Subject: Re: Kernel crash after using new Intel NIC (igb)

Le mardi 24 mai 2011 à 23:06 -0700, Arun Sharma a écrit :
> On Wed, May 25, 2011 at 04:44:29AM +0200, Eric Dumazet wrote:
> > 
> > Hmm, thanks for the report. Are you running x86 or another arch ?
> > 
> 
> This was on x86.
> 
> > We probably need some sort of memory barrier.
> > 
> > However, locking this central lock makes the thing too slow, I'll try to
> > use an atomic_inc_return on p->refcnt instead. (and then lock
> > unused_peers.lock if we got a 0->1 transition)
> 
> Another possibility is to do the list_empty() check twice. Once without
> taking the lock and again with the spinlock held.
> 

Why ?

list_del_init(&p->unused); (done under lock of course) is safe, you can
call it twice, no problem.

No, the real problem is the  (!list_empty(&p->unused) test : It seems to
not always tell the truth if not done under lock.



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