Re: RFC: hold i_rwsem until aio completes

[Christoph Hellwig]

On Thu, Jan 16, 2020 at 03:00:04PM +0100, Jan Kara wrote:
> I'd like to note that using i_dio_count has also one advantage you didn't
> mention. For AIO case, if you need to hold i_rwsem in exclusive mode,
> holding the i_rwsem just for submission part is a significant performance
> advantage (shorter lock hold times allow for higher IO parallelism). I
> guess this could be mitigated by downgrading the lock to shared mode
> once the IO is submitted. But there will be still some degradation visible
> for the cases of mixed exclusive and shared acquisitions because shared
> holders will be blocking exclusive ones for longer time.
> 
> This may be especially painful for filesystems that don't implement DIO
> overwrites with i_rwsem in shared mode...

True.  Fortunately there are patches for ext4 out to move over to that
scheme.  gfs2 will need a little more attention, but that also for other
reasons.