linux-kernel - Re: [PATCH] fix theoretical ccids_{read,write}

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Message-Id: <20070721122149.62c73ba5.akpm@linux-foundation.org>
Date:	Sat, 21 Jul 2007 12:21:49 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix theoretical ccids_{read,write}_lock() race

On Sat, 21 Jul 2007 23:11:04 +0400 Oleg Nesterov <oleg@...sign.ru> wrote:

> On 07/21, Andrew Morton wrote:
> >
> > On Sat, 21 Jul 2007 19:02:06 +0400 Oleg Nesterov <oleg@...sign.ru> wrote:
> > 
> > > Make sure that spin_unlock_wait() is properly ordered wrt atomic_inc().
> > > 
> > > Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
> > > 
> > > --- t/net/dccp/ccid.c~ccid	2006-12-18 18:17:31.000000000 +0300
> > > +++ t/net/dccp/ccid.c	2007-07-21 18:29:21.000000000 +0400
> > > @@ -40,6 +40,7 @@ static inline void ccids_write_unlock(vo
> > >  static inline void ccids_read_lock(void)
> > >  {
> > >  	atomic_inc(&ccids_lockct);
> > > +	smp_mb__after_atomic_inc();
> > >  	spin_unlock_wait(&ccids_lock);
> > >  }
> > >  
> > 
> > Why not just use standard rwlocks in there?
> > 
> > (This is probably an FAQ, but it should be).
> 
> Perhaps because read_lock() doesn't allow to sleep?
> 

down_read() does.

afaict the code doesn't sleep while holding that lock anwyay.
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