Re: [PATCH v9 2/2] blk-mq: Rework blk-mq timeout handling again

[Bart Van Assche]

On Wed, 2018-05-16 at 14:51 +0200, Christoph Hellwig wrote:
> I've been looking at this carefully, and I don't think we need cmpxchg64
> at all, and we don't need anywhere near as many cmpxchg operations either.
> 
> The only reason to include the deadline in the atomic operation is the
> blk_abort_request case, as the blk_mq_add_timer never modifies the
> deadline of a request that someone could be racing with.  So if we
> introduce a new aborted state for use by blk_abort_request we can modify
> the deadline separately (and in fact have a common field with the legacy
> path).

There is another reason the deadline is included in the atomic operation,
namely to handle races between the BLK_EH_RESET_TIMER case in blk_mq_rq_timed_out()
and blk_mq_complete_request(). I don't think that race is addressed properly by
your patch. I will see what I can do to address that race without using 64-bit
atomic operations.

Bart.