Messages in this thread | | | Subject | Re: [PATCH -v2] blk-mq: Start to fix memory ordering... | From | Jens Axboe <> | Date | Wed, 4 Oct 2017 11:18:53 -0600 |
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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
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