linux-kernel - Re: Race between RCU and rmmod

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Message-ID: <20070502120051.GA13349@in.ibm.com>
Date:	Wed, 2 May 2007 17:30:51 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	rusty@...tcorp.com.au, torvalds@...l.org, akpm@...l.org,
	linux-kernel@...r.kernel.org
Subject: Re: Race between RCU and rmmod

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.

Thanks
Dipankar
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