netdev - Re: [PATCH] X25: Replace BKL in sockopts calls

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Date:	Mon, 10 May 2010 06:50:40 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	andrew.hendry@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH] X25: Replace BKL in sockopts calls

Le dimanche 09 mai 2010 à 18:46 -0700, David Miller a écrit :
> From: Andrew Hendry <andrew.hendry@...il.com>
> Date: Sun, 09 May 2010 22:45:23 +1000
> 
> > @@ -465,20 +464,20 @@ static int x25_setsockopt(struct socket *sock, int level, int optname,
> >  	if (get_user(opt, (int __user *)optval))
> >  		goto out;
> >  
> > +	lock_sock(sk);
> >  	x25_sk(sk)->qbitincl = !!opt;
> > +	release_sock(sk);
> 
> This is completely bogus.
> 
> A store is always atomic on an SMP system, and "opt" is in a local variable
> rather than being computed based upon some memory values.
> 
> There is no reason to require locking for this operation.

Well, its probably better than lock_kernel() ;)

qbitincl is a char, I suspect some arches cant store a char in an atomic
way ? Alpha comes to mind.


We now have lock_sock_bh()/unlock_sock_bh() for this kind of very short
sections, where we cant sleep.



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