Re: [PATCH v2] blk-mq: release scheduler resource when request complete

[Jens Axboe]

On 8/13/23 9:23 AM, chengming.zhou@xxxxxxxxx wrote:
> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> 
> Chuck reported [1] a IO hang problem on NFS exports that reside on SATA
> devices and bisected to commit 615939a2ae73 ("blk-mq: defer to the normal
> submission path for post-flush requests").
> 
> We analysed the IO hang problem, found there are two postflush requests
> are waiting for each other.
> 
> The first postflush request completed the REQ_FSEQ_DATA sequence, so go to
> the REQ_FSEQ_POSTFLUSH sequence and added in the flush pending list, but
> failed to blk_kick_flush() because of the second postflush request which
> is inflight waiting in scheduler queue.
> 
> The second postflush waiting in scheduler queue can't be dispatched because
> the first postflush hasn't released scheduler resource even though it has
> completed by itself.
> 
> Fix it by releasing scheduler resource when the first postflush request
> completed, so the second postflush can be dispatched and completed, then
> make blk_kick_flush() succeed.

Thanks, applied with a bit of commit message massaging and adding a
comment for the newly added WARN_ON_ONCE().

-- 
Jens Axboe