[dm-devel] Why queue_work unneeded for REQUEUE bio

[JeffleXu]

On 11/3/20 9:48 PM, Mike Snitzer wrote:
> On Tue, Nov 03 2020 at  4:23am -0500,
> Jeffle Xu <jefflexu at linux.alibaba.com> wrote:
>
>> Hi Mike,
>>
>> Why queue_work() is unnecessary here for bio with BLK_STS_DM_REQUEUE
>> returned?
>>
>> Thanks
>> Jeffle Xu
>>
>> ---
>>   drivers/md/dm.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
>> index c18fc2548518..ae550daa99b5 100644
>> --- a/drivers/md/dm.c
>> +++ b/drivers/md/dm.c
>> @@ -908,9 +908,11 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
>>   			 * Target requested pushing back the I/O.
>>   			 */
>>   			spin_lock_irqsave(&md->deferred_lock, flags);
>> -			if (__noflush_suspending(md))
>> +			if (__noflush_suspending(md)) {
>>   				/* NOTE early return due to BLK_STS_DM_REQUEUE below */
>>   				bio_list_add_head(&md->deferred, io->orig_bio);
>> +				queue_work(md->wq, &md->work);
>> +			}
>>   			else
>>   				/* noflush suspend was interrupted. */
>>   				io->status = BLK_STS_IOERR;
>> -- 
>> 2.27.0
>>
> For the case you highlighted (BLK_STS_DM_REQUEUE + __noflush_suspending)
> I think the missing queue_work is because we're actively dealing with
> the fact that we do _not_ want to flush IO.  SO kicking the workqueue
> there isn't helpful because it just processes work that cannot be issued
> yet -- the workqueue will be kicked upon resume (see __dm_resume's
> dm_queue_flush).


Got it. Thanks.

If we are in process of DMF_NOFLUSH_SUSPENDING, then the BLK_STS_DM_REQUEUE

may be impacted by the suspending, then re-enqueue the bio to @deferred 
list. Or just pop

out BLK_STS_IOERR error.


-- 
Jeffle
Thanks