Re: [PATCH 3/3] blk-mq: Fix the queue freezing mechanism

[Tejun Heo]

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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