On Thu, Jan 05, 2023 at 09:52:00AM -0800, Nadav Amit wrote: > > > > On Jan 4, 2023, at 7:16 PM, Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > > > On Wed, Jan 04, 2023 at 10:13:05AM -0800, Nadav Amit wrote: > >> > >> > >>> On Jan 3, 2023, at 9:42 PM, Ming Lei <ming.lei@xxxxxxxxxx> wrote: > >>> > >>> On Tue, Jan 03, 2023 at 01:47:37PM -0800, Nadav Amit wrote: > >>>> Hello Ming, > >>>> > >>>> I am trying the ublk and it seems very exciting. > >>>> > >>>> However, I encounter an issue when I remove a ublk device that is mounted or > >>>> in use. > >>>> > >>>> In ublk_ctrl_del_dev(), shouldn’t we *not* wait if ublk_idr_freed() is false? > >>>> It seems to me that it is saner to return -EBUSY in such a case and let > >>>> userspace deal with the results. > >>>> > >>>> For instance, if I run the following (using ubdsrv): > >>>> > >>>> $ mkfs.ext4 /dev/ram0 > >>>> $ ./ublk add -t loop -f /dev/ram0 > >>>> $ sudo mount /dev/ublkb0 tmp > >>>> $ sudo ./ublk del -a > >>>> > >>>> ublk_ctrl_del_dev() would not be done until the partition is unmounted, and you > >>>> can get a splat that is similar to the one below. > >>> > >>> The splat itself can be avoided easily by replace wait_event with > >>> wait_event_timeout() plus loop, but I guess you think the sync delete > >>> isn't good too? > >> > >> I don’t think the splat is the issue. The issue is the blocking behavior, > >> which is both unconditional and unbounded in time, and (worse) takes place > >> without relinquishing the locks. wait_event_timeout() is therefore not a > >> valid solution IMHO. > >> > >>> > >>>> > >>>> What do you say? Would you agree to change the behavior to return -EBUSY? > >>> > >>> It is designed in this way from beginning, and I believe it is just for > >>> the sake of simplicity, and one point is that the device number needs > >>> to be freed after 'ublk del' returns. > >>> > >>> But if it isn't friendly from user's viewpoint, we can change to return > >>> -EBUSY. One simple solution is to check if the ublk block device > >>> is opened before running any deletion action, if yes, stop to delete it > >>> and return -EBUSY; otherwise go ahead and stop & delete the pair of devices. > >>> And the userspace part(ublk utility) needs update too. > >>> > >>> However, -EBUSY isn't perfect too, cause user has to retry the delete > >>> command manually. > >> > >> I understand your considerations. My intuition is that just as umount > >> cannot be done while a file is opened and would return -EBUSY, so should > >> deleting the ublock while the ublk is in use. > >> > >> So as I see it, there are 2 possible options for proper deletion of ublk, > >> and actually both can be implemented and distinguished with a new flag > >> (UBLK_F_*): > >> > >> 1. Blocking - similar to the way it is done today, but (hopefully) without > >> holding locks, and with using wait_event_interruptible() instead of > >> wait_event() to allow interruption (and return EINTR if interrupted). > >> > >> 2. Best-effort - returning EBUSY if it cannot be removed. > >> > >> I can imagine use-cases for both, and it would also allow you not to > >> change ubdsrv if you choose so. > >> > >> Does it make sense? > > > > I prefer to the 1st approach: > > > > 1) the wait event is still one positive signal for user to cleanup the > > device use, since the correct step is to umount ublk disk before deleting > > the device. > > > > 2) the wait still can avoid the current context to reuse the device > > number > > > > 3) after switching to wait_event_interruptible(), we need to avoid > > double delete, and one flag of UB_STATE_DELETED can be used for failing > > new delete command. > > > > 4) IMO new flag(UBLK_F_*) isn't needed to distinguish this change > > with current behavior. > > > > Please let us know if you'd like to cook one patch for improving > > the delete handling. > > I can take a stab on it, but only in about 2 weeks time. > > > > > BTW, there could be another option, such as, 'ublk delete --no-wait' just > > run the remove and without waiting at all, but not sure if it is useful. > > > > I considered the userspace ublk as one possible implementation. I am not > sure this affects the kernel interfaces that are needed. ublk driver needs to be told if --no-wait is applied, so either one UBLK_F_* or a parameter of UBLK_CMD_DEL_DEV is needed. Thanks, Ming
