netdev - Re: Warning in net/ipv4/af

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Message-ID: <1274934075.2542.28.camel@edumazet-laptop>
Date:	Thu, 27 May 2010 06:21:15 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	anton@...ba.org, netdev@...r.kernel.org
Subject: Re: Warning in net/ipv4/af_inet.c:154

Le mercredi 26 mai 2010 à 21:06 -0700, David Miller a écrit :
> From: Anton Blanchard <anton@...ba.org>
> Date: Thu, 27 May 2010 13:56:17 +1000
> 
> > I'm somewhat confused by the two stage locking in the socket lock
> > (ie sk_lock.slock and sk_lock.owned).
> > 
> > What state should the socket lock be in for serialising updates of
> > sk_forward_alloc? In some cases we appear to update it with sk_lock.slock =
> > unlocked, sk_lock.owned = 1:
> 
> If sh_lock.owned=1 the user has grabbed exclusive sleepable lock on the
> socket, it does this via something approximating:
> 
> retry:
> 	spin_lock(&sk_lock.slock);
> 	was_locked = sk_lock.owned;
> 	if (!was_locked)
> 		sk_lock.owned = 1;
> 	spin_unlock(&sk_lock.slock);
> 	if (was_locked) {
> 		sleep_on(condition(sk_lock.owned));
> 		goto retry;
> 	}
> 
> This allows user context code to sleep with exclusive access to the
> socket.
> 
> Code that cannot sleep takes the spinlock, and queues the work if the
> owner field if non-zero.  Else, it keeps the spinlock held and does
> the work.
> 
> In either case, socket modifications are thus done completely protected
> from other contexts.
> 
> 

Yes, but not on the case one user context uses the 'slow locking', and
another uses the 'fast locking'.

I am working on a patch now.




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