Messages in this thread | | | Date | Thu, 24 Sep 2015 13:49:38 -0400 | From | Tejun Heo <> | Subject | Re: [PATCH 3/3] blk-mq: Fix the queue freezing mechanism |
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Hello, Bart.
On Thu, Sep 24, 2015 at 10:35:41AM -0700, Bart Van Assche wrote: > My interpretation of the percpu_ref_tryget_live() implementation in > <linux/percpu-refcount.h> is that the tryget operation will only fail if the > refcount is in atomic mode and additionally the __PERCPU_REF_DEAD flag has > been set.
Yeah and percpu_ref_kill() does both.
> >Also, what does the barriers do in your patch? > > My intention was to guarantee that on architectures that do not provide the > same ordering guarantees as x86 (e.g. PPC or ARM) that the store and load > operations on mq_freeze_depth and mq_usage_counter would not be reordered. > However, it is probably safe to leave out the barrier I proposed to > introduce in blk_mq_queue_enter() since it is acceptable that there is some > delay in communicating mq_freeze_depth updates from the CPU that modified > that counter to the CPU that reads that counter.
Hmmm... please don't use barriers this way. Use it only when there's a clear requirement for interlocking writer and reader pair. There isn't one here. All it does is confusing people trying to read the code.
> >The only race condition that I can see there is if unfreeze and freeze > >race each other and freeze tries to kill the ref which hasn't finished > >reinit yet. We prolly want to put mutexes around freeze/unfreeze so > >that they're serialized if something like that can happen (it isn't a > >hot path to begin with). > > My concern is that the following could happen if mq_freeze_depth is not > checked in the hot path of blk_mq_queue_enter(): > * mq_usage_counter >= 1 before blk_mq_freeze_queue() is called. > * blk_mq_freeze_queue() keeps waiting forever if new requests are queued > faster than that these requests complete.
Again, that doesn't happen.
Thanks.
-- tejun
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