linux-kernel - Re: Race between RCU and rmmod

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Message-Id: <1178109221.28659.312.camel@localhost.localdomain>
Date:	Wed, 02 May 2007 22:33:40 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	dipankar@...ibm.com
Cc:	David Howells <dhowells@...hat.com>, torvalds@...l.org,
	akpm@...l.org, linux-kernel@...r.kernel.org
Subject: Re: Race between RCU and rmmod

On Wed, 2007-05-02 at 17:30 +0530, Dipankar Sarma 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.

Hi David, Dipankar,

	My first thought was wondering if doing it for them isn't a bad thing,
but they need to do it if they've got other teardown which would mess up
RCU callbacks anyway...

Cheers,
Rusty.


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