linux-kernel - Re: [PATCH -v2] blk-mq: Start to fix memory ordering...

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Date:   Wed, 4 Oct 2017 11:18:53 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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,
        stern@...land.harvard.edu
Subject: Re: [PATCH -v2] blk-mq: Start to fix memory ordering...

On 09/06/2017 02:00 AM, 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 ;-)

Sorry for the belated response on this, I spent some time and looked
over everything. Looks solid to me.

I'll queue this up for some testing, and also add a compile check to
prevent us violating the need to have STARTED and COMPLETED be in
the same byte of storage.

-- 
Jens Axboe