> 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.
