netdev - Re: Crypto oops in async_chainiv_do

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Date:	Thu, 3 Sep 2009 11:53:44 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Brad Bosch <bradbosch@...cast.net>
Cc:	linux-crypto@...r.kernel.org, netdev@...r.kernel.org,
	offbase0@...il.com
Subject: Re: Crypto oops in async_chainiv_do_postponed

On Wed, Sep 02, 2009 at 06:47:49PM -0500, Brad Bosch wrote:
>
> Assume the worker thread is executing between the dequeue in
> async_chainiv_do_postponed and the clear_bit call in
> async_chainiv_schedule_work.  Further assume that we are processing
> the last item on the queue so durring this time, ctx->queue.qlen =
> 0.  **INUSE is still set at this point.
> 
> Meanwhile, three threads enter async_chainiv_givencrypt for the same
> ctx at about the same time.
> 
> Thread one calls test_and_set_bit which returns 1 and calls
> async_cahiniv_postpone_request but suppose it has not yet enqueued.
> Now INUSE is set and qlen=0.
> 
> Next, the worker thread calls clear_bit in async_chainiv_schedule_work
> but it is interrupted before it can call test_and_set_bit.  Now INUSE
> is clear and qlen=0
> 
> The test_and_set_bit in thread two is called at this moment and
> returns 0 and then calls async_chainiv_givencrypt_tail.  Now INUSE is
> set and qlen=0.
> 
> Thread one now locks the ctx and calls skcipher_enqueue_givcrypt and
> unlocks.  Now INUSE is set and qlen=1.
> 
> Thread three calls test_and_set_bit which returns 1 and then it clears
> INUSE since qlen=1 and it calls postpone with INUSE clear and qlen=1

How can thread three clear INUSE if test_and_set_bit returned 1?
If thread three sees it set then it will postpone.  It can only
clear it if it was not set originally.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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