linux-kernel - Re: Race between RCU and rmmod

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Date:	Wed, 2 May 2007 11:47:51 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	dipankar@...ibm.com
Cc:	David Howells <dhowells@...hat.com>, rusty@...tcorp.com.au,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: Race between RCU and rmmod

On Wed, 2 May 2007 17:30:51 +0530
Dipankar Sarma <dipankar@...ibm.com> wrote:

> On Wed, May 02, 2007 at 12:50:24PM +0100, David Howells wrote:
> > 
> > Hi Dipankar, Rusty,
> > 
> > I seem to have found a race between RCU and rmmod.  What I see appears to be
> > an RCU destructor function that has a call pending but lives in a module, gets
> > deleted before the RCU callback is processed:
> > 
> > RIP: 0010:[<ffffffff880329b7>]  [<ffffffff880329b7>]
> > 
> > I think that rmmod needs to clear the RCU destructor queue, probably inside of
> > __try_stop_module().
> 
> This is why we have rcu_barrier() although the corresponding documentation
> patch seems to have got dropped. Modules that use RCU must call
> rcu_barrier() in their cleanup routine.
> 

hm, I never knew that.

akpm:/usr/src/linux-2.6.21> grep -r rcu_ drivers | wc -l
182
akpm:/usr/src/linux-2.6.21> grep -r rcu_barrier drivers | wc -l 
0

For a start we should undrop that documentation patch, please.
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