On 2022/7/14 18:48, Ming Lei wrote: > 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. > Thanks, Ming. I understand the aborting code now. And it looks good to me. Previously I think maybe monitor_work and task_work may be scheduled at the same time while task is exiting and blk_mq_end_request() on the same tag could be called twice. But I find there is a check on ublk_io's flag(UBLK_IO_FLAG_ACTIVE) in ublk_daemon_monitor_work() and ublk_io is aborted in task_work immediately(with UBLK_IO_FLAG_ACTIVE set, not cleared yet) So there is no chance to call a send blk_mq_end_request() on the same tag. Besides, for ublk_ios with UBLK_IO_FLAG_ACTIVE unset, stop_work scheduled in monitor work will call ublk_cancel_queue() by sending cqes with UBLK_IO_RES_ABORT. Put it together: When daemon is PF_EXITING: 1) current ublk_io: aborted immediately in task_work 2) UBLK_IO_FLAG_ACTIVE set: aborted in ublk_daemon_monitor_work 3) UBLK_IO_FLAG_ACTIVE unset: send cqe with UBLK_IO_RES_ABORT Hope I'm correct this time. :) >> >> >> [...] >> >>> + >>> +/* >>> + * __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. Ok, I see. > >> >> 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. Ok, I see. I think if one ubq daemon is killed(and blk-mq requests related to it are aborted), stop work should call del_gendisk() and other ubq daemons can still complete blk-mq requests but no new blk-mq requests will be issued. After that, these unkilled ubq daemons will get UBLK_IO_RES_ABORT cqes and exit by themselves. > > > Thanks, > Ming
