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Date:   Wed, 6 Sep 2017 10:02:00 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Jens Axboe <axboe@...com>, <parri.andrea@...il.com>,
        <linux-kernel@...r.kernel.org>, <tom.leiming@...il.com>,
        <hch@....de>, <paulmck@...ux.vnet.ibm.com>, <will.deacon@....com>,
        <boqun.feng@...il.com>
Subject: Re: [PATCH -v2] blk-mq: Start to fix memory ordering...

On Wed, 6 Sep 2017, Peter Zijlstra wrote:

> Attempt to untangle the ordering in blk-mq. The patch introducing the
> single smp_mb__before_atomic() is obviously broken in that it doesn't
> clearly specify a pairing barrier and an obtained guarantee.
> 
> The comment is further misleading in that it hints that the
> deadline store and the COMPLETE store also need to be ordered, but
> AFAICT there is no such dependency. However what does appear to be
> important is the clear happening _after_ the store, and that worked by
> pure accident.
> 
> This clarifies blk_mq_start_request() -- we should not get there with
> STARTING set -- this simplifies the code and makes the barrier usage
> sane (the old code could be read to allow not having _any_ atomic after
> the barrier, in which case the barrier hasn't got anything to order). We
> then also introduce the missing pairing barrier for it.
> 
> Also down-grade the barrier to smp_wmb(), this is cheaper for
> PowerPC/ARM and doesn't cost anything extra on x86.
> 
> And it documents the STARTING vs COMPLETE ordering. Although I've not
> been entirely successful in reverse engineering the blk-mq state
> machine so there might still be more funnies around timeout vs
> requeue.
> 
> If I got anything wrong, feel free to educate me by adding comments to
> clarify things ;-)
> 
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Ming Lei <tom.leiming@...il.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Jens Axboe <axboe@...com>
> Cc: Andrea Parri <parri.andrea@...il.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Bart Van Assche <bart.vanassche@....com>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Fixes: 538b75341835 ("blk-mq: request deadline must be visible before marking rq as started")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  - spelling; Andrea and Bart
>  - compiles (urgh!)
>  - smp_wmb(); Adrea
> 
> 
>  block/blk-mq.c      | 52 ++++++++++++++++++++++++++++++++++++++++------------
>  block/blk-timeout.c |  2 +-
>  2 files changed, 41 insertions(+), 13 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4603b115e234..506a0f355117 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -558,22 +558,32 @@ void blk_mq_start_request(struct request *rq)
>  
>  	blk_add_timer(rq);
>  
> -	/*
> -	 * Ensure that ->deadline is visible before set the started
> -	 * flag and clear the completed flag.
> -	 */
> -	smp_mb__before_atomic();
> +	WARN_ON_ONCE(test_bit(REQ_ATOM_STARTED, &rq->atomic_flags));
>  
>  	/*
>  	 * Mark us as started and clear complete. Complete might have been
>  	 * set if requeue raced with timeout, which then marked it as
>  	 * complete. So be sure to clear complete again when we start
>  	 * the request, otherwise we'll ignore the completion event.
> +	 *
> +	 * Ensure that ->deadline is visible before we set STARTED, such that
> +	 * blk_mq_check_expired() is guaranteed to observe our ->deadline when
> +	 * it observes STARTED.
>  	 */
> -	if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
> -		set_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
> -	if (test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags))
> +	smp_wmb();
> +	set_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
> +	if (test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags)) {
> +		/*
> +		 * Coherence order guarantees these consecutive stores to a
> +		 * single variable propagate in the specified order. Thus the
> +		 * clear_bit() is ordered _after_ the set bit. See
> +		 * blk_mq_check_expired().
> +		 *
> +		 * (the bits must be part of the same byte for this to be
> +		 * true).

Adding this comment is well and good, but for more security you should 
also add a comment (maybe even a compile-time check) to the place where 
REQ_ATOM_STARTED and REQ_ATOM_COMPLETE are defined.  Otherwise they 
might eventually get moved into separate bytes.

Alan Stern

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