On Wed, Jul 20, 2022 at 11:00:40AM +0200, Christoph Hellwig wrote: > On Wed, Jul 20, 2022 at 03:47:27PM +0800, Ming Lei wrote: > > > What is broken in START_DEV/STOP_DEV? Please explain the semantics you > > > want and what doesn't work. FYI, there is nothing in the test suite the > > > complains. And besides the obvious block layer bug that Jens found you > > > seemed to be perfectly happy with the semantics. > > > > START_DEV calls add_disk(), and STOP_DEV calls del_gendisk(), but if > > GD_OWNS_QUEUE is set, blk_mq_exit_queue() will be called in > > del_gendisk(), then the following START_DEV will stuck. > > Uh, yeah. alloc_disk and add_disk are supposed to be paired and > not split over different ioctls. The lifetime rules here are > rather broken. Can you explain what this way breaks? And why can't one disk be added/deleted multiple times? > > > > > similar with scsi's, in which disk rebind needs to be supported > > > > and GD_OWNS_QUEUE can't be set. > > > > > > SCSI needs it because it needs the request_queue to probe for what ULP > > > to bind to, and it allows to unbind the ULP. None of that is the case > > > here. And managing the lifetimes separately is a complete mess, so > > > don't do it. Especially not in a virtual driver where you don't have > > > to cater to a long set protocol like SCSI. > > > > If blk_mq_exit_queue is called in del_gendisk() for scsi, how can > > re-bind work as expected since it needs one completely workable > > request queue instead of partial exited one? > > For !GD_OWNS_QUEUE blk_mq_exit_queue is not called from del_gendisk(). That is why scsi can't set GD_OWNS_QUEUE, for any driver, if disk rebind or similar behavior is needed, GD_OWNS_QUEUE can't be set. That is why ublk_drv uses separated queue/disk. thanks, Ming
