Re: [dm-devel] [PATCH 4/4] dm: support I/O polling

[Jeff Moyer <jmoyer@redhat.com>]

Hi, Mikulas,

Mikulas Patocka <mpatocka@redhat.com> writes:

> On Wed, 3 Mar 2021, JeffleXu wrote:
>
>> 
>> 
>> On 3/3/21 3:05 AM, Mikulas Patocka wrote:
>> 
>> > Support I/O polling if submit_bio_noacct_mq_direct returned non-empty
>> > cookie.
>> > 
>> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
>> > 
>> > ---
>> >  drivers/md/dm.c |    5 +++++
>> >  1 file changed, 5 insertions(+)
>> > 
>> > Index: linux-2.6/drivers/md/dm.c
>> > ===================================================================
>> > --- linux-2.6.orig/drivers/md/dm.c	2021-03-02 19:26:34.000000000 +0100
>> > +++ linux-2.6/drivers/md/dm.c	2021-03-02 19:26:34.000000000 +0100
>> > @@ -1682,6 +1682,11 @@ static void __split_and_process_bio(stru
>> >  		}
>> >  	}
>> >  
>> > +	if (ci.poll_cookie != BLK_QC_T_NONE) {
>> > +		while (atomic_read(&ci.io->io_count) > 1 &&
>> > +		       blk_poll(ci.poll_queue, ci.poll_cookie, true)) ;
>> > +	}
>> > +
>> >  	/* drop the extra reference count */
>> >  	dec_pending(ci.io, errno_to_blk_status(error));
>> >  }
>> 
>> It seems that the general idea of your design is to
>> 1) submit *one* split bio
>> 2) blk_poll(), waiting the previously submitted split bio complets
>
> No, I submit all the bios and poll for the last one.

What happens if the last bio completes first?  It looks like you will
call blk_poll with a cookie that already completed, and I'm pretty sure
that's invalid.

Thanks,
Jeff