On Thu, Jul 14, 2022 at 06:20:38PM +0800, Ziyang Zhang wrote: > On 2022/7/13 22:07, Ming Lei wrote: > > This is the driver part of userspace block driver(ublk driver), the other > > part is userspace daemon part(ublksrv)[1]. > > > > The two parts communicate by io_uring's IORING_OP_URING_CMD with one > > shared cmd buffer for storing io command, and the buffer is read only for > > ublksrv, each io command is indexed by io request tag directly, and > > is written by ublk driver. > > > > For example, when one READ io request is submitted to ublk block driver, ublk > > driver stores the io command into cmd buffer first, then completes one > > IORING_OP_URING_CMD for notifying ublksrv, and the URING_CMD is issued to > > ublk driver beforehand by ublksrv for getting notification of any new io request, > > and each URING_CMD is associated with one io request by tag. > > > > After ublksrv gets the io command, it translates and handles the ublk io > > request, such as, for the ublk-loop target, ublksrv translates the request > > into same request on another file or disk, like the kernel loop block > > driver. In ublksrv's implementation, the io is still handled by io_uring, > > and share same ring with IORING_OP_URING_CMD command. When the target io > > request is done, the same IORING_OP_URING_CMD is issued to ublk driver for > > both committing io request result and getting future notification of new > > io request. > > > > Another thing done by ublk driver is to copy data between kernel io > > request and ublksrv's io buffer: > > > > 1) before ubsrv handles WRITE request, copy the request's data into > > ublksrv's userspace io buffer, so that ublksrv can handle the write > > request > > > > 2) after ubsrv handles READ request, copy ublksrv's userspace io buffer > > into this READ request, then ublk driver can complete the READ request > > > > Zero copy may be switched if mm is ready to support it. > > > > ublk driver doesn't handle any logic of the specific user space driver, > > so it is small/simple enough. > > > > [1] ublksrv > > > > https://github.com/ming1/ubdsrv > > > > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> > > --- > > > Hi, Ming > > I find that a big change from v4 to v5 is the simplification of locks. > > In v5 you remove ubq->abort_lock, and I want to ask why it is OK to remove it? Actually V4 and previous version dealt with the issue too complicated. > > If you have time, could you explain how ublk deals with potential race on: > 1)queue_rq 2)ublk_abort_queue 3) ublk_ctrl_stop_dev 4) ublk_rq_task_work. > (Lock in ublk really confuses me...) One big change is the following code: __ublk_rq_task_work(): bool task_exiting = current != ubq->ubq_daemon || (current->flags & PF_EXITING); ... if (unlikely(task_exiting)) { blk_mq_end_request(req, BLK_STS_IOERR); mod_delayed_work(system_wq, &ub->monitor_work, 0); return; } Abort is always started after PF_EXITING is set, but if PF_EXITING is set, __ublk_rq_task_work fails the request immediately, then io->flags won't be touched, then no race with abort. Also PF_EXITING is per-task flag, can only be set before calling __ublk_rq_task_work(), and setting it actually serialized with calling task work func. In ublk_queue_rq(), we don't touch io->flags, so there isn't race with abort. Wrt. ublk_ctrl_stop_dev(), it isn't related with abort directly, and if del_gendisk() waits for inflight IO, abort work will be started for making forward progress. After del_gendisk() returns, there can't be any inflight io, so it is safe to cancel other pending io command. > > > [...] > > > + > > +/* > > + * __ublk_fail_req() may be called from abort context or ->ubq_daemon > > + * context during exiting, so lock is required. > > + * > > + * Also aborting may not be started yet, keep in mind that one failed > > + * request may be issued by block layer again. > > + */ > > +static void __ublk_fail_req(struct ublk_io *io, struct request *req) > > +{ > > + WARN_ON_ONCE(io->flags & UBLK_IO_FLAG_ACTIVE); > > + > > + if (!(io->flags & UBLK_IO_FLAG_ABORTED)) { > > + io->flags |= UBLK_IO_FLAG_ABORTED; > > + blk_mq_end_request(req, BLK_STS_IOERR); > > + } > > +} > > + > > [...] > > > + > > +/* > > + * When ->ubq_daemon is exiting, either new request is ended immediately, > > + * or any queued io command is drained, so it is safe to abort queue > > + * lockless > > + */ > > +static void ublk_abort_queue(struct ublk_device *ub, struct ublk_queue *ubq) > > +{ > > + int i; > > + > > + if (!ublk_get_device(ub)) > > + return; > > + > > + for (i = 0; i < ubq->q_depth; i++) { > > + struct ublk_io *io = &ubq->ios[i]; > > + > > + if (!(io->flags & UBLK_IO_FLAG_ACTIVE)) { > > + struct request *rq; > > + > > + /* > > + * Either we fail the request or ublk_rq_task_work_fn > > + * will do it > > + */ > > + rq = blk_mq_tag_to_rq(ub->tag_set.tags[ubq->q_id], i); > > + if (rq) > > + __ublk_fail_req(io, rq); > > + } > > + } > > + ublk_put_device(ub); > > +} > > + > > > Another problem: > > 1) comment of __ublk_fail_req(): "so lock is required" Yeah, now __ublk_fail_req is only called in abort context, and no race with task work any more, so lock isn't needed. > > 2) comment of ublk_abort_queue(): "so it is safe to abort queue lockless" This comment is updated in v5, and it is correct. > > 3) ublk_abort_queue() calls _ublk_fail_req() on all ubqs. No, ublk_abort_queue() only aborts the passed ubq, so if one ubq daemon is aborted, other ubqs can still handle IO during deleting disk. Thanks, Ming
