netdev - Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry

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Message-ID: <20130529013111.GS6172@linux.vnet.ibm.com>
Date:	Tue, 28 May 2013 18:31:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Roman Gushchin <klamm@...dex-team.ru>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Dipankar Sarma <dipankar@...ibm.com>, zhmurov@...dex-team.ru,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu
 macro

On Tue, May 28, 2013 at 05:34:53PM -0700, Eric Dumazet wrote:
> On Tue, 2013-05-28 at 13:10 +0400, Roman Gushchin wrote:
> > On 28.05.2013 04:12, Eric Dumazet wrote:
> 
> > > Adding a barrier() is probably what we want.
> > 
> > I agree, inserting barrier() is also a correct and working fix.
> 
> Yeah, but I can not find a clean way to put it inside the "for (;;)"
> 
> for (barrier();;)  ->
> 
> error: expected expression before ‘__asm__’
> 
> No user currently does :
> 
> if (condition)
> 	hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)
> 
> But who knows...

I still have my earlier question, but I suggest "({ barrier(); XXX })"
to put the barrier into the for loop, either in the second or third
clause, where XXX was the original second or third clause.

							Thanx, Paul

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