Date:   Fri, 15 Jul 2022 10:04:57 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Ziyang Zhang <ZiyangZhang@...ux.alibaba.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        io-uring@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Subject: Re: [PATCH V5 1/2] ublk_drv: add io_uring based userspace block
 driver

On Thu, Jul 14, 2022 at 09:23:40PM +0800, Ziyang Zhang wrote:
> 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@...hat.com>
> >>> ---
> >>
> >>
> >> 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

Precisely it is just that ublk io request is ended immediately, so io->flags
won't be touched.

> 
> 2) UBLK_IO_FLAG_ACTIVE set: aborted in ublk_daemon_monitor_work

This part is important for making forward progress, that is why it has
to be done in a wq context.

> 
> 3) UBLK_IO_FLAG_ACTIVE unset: send cqe with UBLK_IO_RES_ABORT

This is the 2nd stage of aborting after disk is deleted.

In short, it is one two-stage aborting, and the idea is actually
straightforward.

> 
> 
> Hope I'm correct this time. :)

Absolutely.

> 
> >>
> >>
> >> [...]
> >>
> >>> +
> >>> +/*
> >>> + * __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.

Yeah, you are right.


Thanks,
Ming

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