Re: [PATCH v3] ublk: improve detection and handling of ublk server exit

Ming Lei
2025-04-08T03:06:45Z

On Mon, Apr 07, 2025 at 01:16:33PM -0600, Uday Shankar wrote:
> On Sat, Apr 05, 2025 at 10:59:29PM +0800, Ming Lei wrote:
> > On Thu, Apr 03, 2025 at 06:05:57PM -0600, Uday Shankar wrote:
> > > There are currently two ways in which ublk server exit is detected by
> > > ublk_drv:
> > >
> > > 1. uring_cmd cancellation. If there are any outstanding uring_cmds which
> > > have not been completed to the ublk server when it exits, io_uring
> > > calls the uring_cmd callback with a special cancellation flag as the
> > > issuing task is exiting.
> > > 2. I/O timeout. This is needed in addition to the above to handle the
> > > "saturated queue" case, when all I/Os for a given queue are in the
> > > ublk server, and therefore there are no outstanding uring_cmds to
> > > cancel when the ublk server exits.
> > >
> > > There are a couple of issues with this approach:
> > >
> > > - It is complex and inelegant to have two methods to detect the same
> > > condition
> > > - The second method detects ublk server exit only after a long delay
> > > (~30s, the default timeout assigned by the block layer). This delays
> > > the nosrv behavior from kicking in and potential subsequent recovery
> > > of the device.
> > >
> > > The second issue is brought to light with the new test_generic_04. It
> > > fails before this fix:
> > >
> > > selftests: ublk: test_generic_04.sh
> > > dev id is 0
> > > dd: error writing '/dev/ublkb0': Input/output error
> > > 1+0 records in
> > > 0+0 records out
> > > 0 bytes copied, 30.0611 s, 0.0 kB/s
> > > DEAD
> > > dd took 31 seconds to exit (>= 5s tolerance)!
> > > generic_04 : [FAIL]
> > >
> > > Fix this by instead detecting and handling ublk server exit in the
> > > character file release callback. This has several advantages:
> > >
> > > - This one place can handle both saturated and unsaturated queues. Thus,
> > > it replaces both preexisting methods of detecting ublk server exit.
> > > - It runs quickly on ublk server exit - there is no 30s delay.
> > > - It starts the process of removing task references in ublk_drv. This is
> > > needed if we want to relax restrictions in the driver like letting
> > > only one thread serve each queue
> > >
> > > There is also the disadvantage that the character file release callback
> > > can also be triggered by intentional close of the file, which is a
> > > significant behavior change. Preexisting ublk servers (libublksrv) are
> > > dependent on the ability to open/close the file multiple times. To
> > > address this, only transition to a nosrv state if the file is released
> > > while the ublk device is live. This allows for programs to open/close
> > > the file multiple times during setup. It is still a behavior change if a
> > > ublk server decides to close/reopen the file while the device is LIVE
> > > (i.e. while it is responsible for serving I/O), but that would be highly
> > > unusual. This behavior is in line with what is done by FUSE, which is
> > > very similar to ublk in that a userspace daemon is providing services
> > > traditionally provided by the kernel.
> > >
> > > With this change in, the new test (and all other selftests, and all
> > > ublksrv tests) pass:
> > >
> > > selftests: ublk: test_generic_04.sh
> > > dev id is 0
> > > dd: error writing '/dev/ublkb0': Input/output error
> > > 1+0 records in
> > > 0+0 records out
> > > 0 bytes copied, 0.0376731 s, 0.0 kB/s
> > > DEAD
> > > generic_04 : [PASS]
> > >
> > > Signed-off-by: Uday Shankar <ushankar@purestorage.com>
> > > ---
> > > Changes in v3:
> > > - Quiesce queue earlier to avoid concurrent cancellation and "normal"
> > > completion of io_uring cmds (Ming Lei)
> > > - Fix del_gendisk hang, found by test_stress_02
> > > - Remove unnecessary parameters in fault_inject target (Ming Lei)
> > > - Fix delay implementation to have separate per-I/O delay instead of
> > > blocking the whole thread (Ming Lei)
> > > - Add delay_us to docs
> > > - Link to v2: https://lore.kernel.org/r/20250402-ublk_timeout-v2-1-249bc5523000@purestorage.com
> > >
> > > Changes in v2:
> > > - Leave null ublk selftests target untouched, instead create new
> > > fault_inject target for injecting per-I/O delay (Ming Lei)
> > > - Allow multiple open/close of ublk character device with some
> > > restrictions
> > > - Drop patches which made it in separately at https://lore.kernel.org/r/20250401-ublk_selftests-v1-1-98129c9bc8bb@purestorage.com
> > > - Consolidate more nosrv logic in ublk character device release, and
> > > associated code cleanup
> > > - Link to v1: https://lore.kernel.org/r/20250325-ublk_timeout-v1-0-262f0121a7bd@purestorage.com
> > > ---
> > > drivers/block/ublk_drv.c | 228 +++++++++---------------
> > > tools/testing/selftests/ublk/Makefile | 4 +-
> > > tools/testing/selftests/ublk/fault_inject.c | 72 ++++++++
> > > tools/testing/selftests/ublk/kublk.c | 6 +-
> > > tools/testing/selftests/ublk/kublk.h | 4 +
> > > tools/testing/selftests/ublk/test_generic_04.sh | 43 +++++
> > > 6 files changed, 215 insertions(+), 142 deletions(-)
> > >
> > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > > index 2fd05c1bd30b03343cb6f357f8c08dd92ff47af9..73baa9d22ccafb00723defa755a0b3aab7238934 100644
> > > --- a/drivers/block/ublk_drv.c
> > > +++ b/drivers/block/ublk_drv.c
> > > @@ -162,7 +162,6 @@ struct ublk_queue {
> > >
> > > bool force_abort;
> > > bool timeout;
> > > - bool canceling;
> > > bool fail_io; /* copy of dev->state == UBLK_S_DEV_FAIL_IO */
> > > unsigned short nr_io_ready; /* how many ios setup */
> > > spinlock_t cancel_lock;
> > > @@ -199,8 +198,6 @@ struct ublk_device {
> > > struct completion completion;
> > > unsigned int nr_queues_ready;
> > > unsigned int nr_privileged_daemon;
> > > -
> > > - struct work_struct nosrv_work;
> > > };
> > >
> > > /* header of ublk_params */
> > > @@ -209,8 +206,9 @@ struct ublk_params_header {
> > > __u32 types;
> > > };
> > >
> > > -static bool ublk_abort_requests(struct ublk_device *ub, struct ublk_queue *ubq);
> > > -
> > > +static void ublk_stop_dev_unlocked(struct ublk_device *ub);
> > > +static void ublk_abort_queue(struct ublk_device *ub, struct ublk_queue *ubq);
> > > +static void __ublk_quiesce_dev(struct ublk_device *ub);
> > > static inline struct request *__ublk_check_and_get_req(struct ublk_device *ub,
> > > struct ublk_queue *ubq, int tag, size_t offset);
> > > static inline unsigned int ublk_req_build_flags(struct request *req);
> > > @@ -1314,8 +1312,6 @@ static void ublk_queue_cmd_list(struct ublk_queue *ubq, struct rq_list *l)
> > > static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > > {
> > > struct ublk_queue *ubq = rq->mq_hctx->driver_data;
> > > - unsigned int nr_inflight = 0;
> > > - int i;
> > >
> > > if (ubq->flags & UBLK_F_UNPRIVILEGED_DEV) {
> > > if (!ubq->timeout) {
> > > @@ -1326,26 +1322,6 @@ static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > > return BLK_EH_DONE;
> > > }
> > >
> > > - if (!ubq_daemon_is_dying(ubq))
> > > - return BLK_EH_RESET_TIMER;
> > > -
> > > - for (i = 0; i < ubq->q_depth; i++) {
> > > - struct ublk_io *io = &ubq->ios[i];
> > > -
> > > - if (!(io->flags & UBLK_IO_FLAG_ACTIVE))
> > > - nr_inflight++;
> > > - }
> > > -
> > > - /* cancelable uring_cmd can't help us if all commands are in-flight */
> > > - if (nr_inflight == ubq->q_depth) {
> > > - struct ublk_device *ub = ubq->dev;
> > > -
> > > - if (ublk_abort_requests(ub, ubq)) {
> > > - schedule_work(&ub->nosrv_work);
> > > - }
> > > - return BLK_EH_DONE;
> > > - }
> > > -
> > > return BLK_EH_RESET_TIMER;
> > > }
> > >
> > > @@ -1356,19 +1332,16 @@ static blk_status_t ublk_prep_req(struct ublk_queue *ubq, struct request *rq)
> > > if (unlikely(ubq->fail_io))
> > > return BLK_STS_TARGET;
> > >
> > > - /* With recovery feature enabled, force_abort is set in
> > > - * ublk_stop_dev() before calling del_gendisk(). We have to
> > > - * abort all requeued and new rqs here to let del_gendisk()
> > > - * move on. Besides, we cannot not call io_uring_cmd_complete_in_task()
> > > - * to avoid UAF on io_uring ctx.
> > > + /*
> > > + * force_abort is set in ublk_stop_dev() before calling
> > > + * del_gendisk(). We have to abort all requeued and new rqs here
> > > + * to let del_gendisk() move on. Besides, we cannot not call
> > > + * io_uring_cmd_complete_in_task() to avoid UAF on io_uring ctx.
> > > *
> > > * Note: force_abort is guaranteed to be seen because it is set
> > > * before request queue is unqiuesced.
> > > */
> > > - if (ublk_nosrv_should_queue_io(ubq) && unlikely(ubq->force_abort))
> > > - return BLK_STS_IOERR;
> > > -
> > > - if (unlikely(ubq->canceling))
> > > + if (unlikely(ubq->force_abort))
> > > return BLK_STS_IOERR;
> > >
> > > /* fill iod to slot in io cmd buffer */
> > > @@ -1391,16 +1364,6 @@ static blk_status_t ublk_queue_rq(struct blk_mq_hw_ctx *hctx,
> > > if (res != BLK_STS_OK)
> > > return res;
> > >
> > > - /*
> > > - * ->canceling has to be handled after ->force_abort and ->fail_io
> > > - * is dealt with, otherwise this request may not be failed in case
> > > - * of recovery, and cause hang when deleting disk
> > > - */
> > > - if (unlikely(ubq->canceling)) {
> > > - __ublk_abort_rq(ubq, rq);
> > > - return BLK_STS_OK;
> > > - }
> > > -
> >
> > If `ubq->canceling` is removed, how to handle recovery? The request has to
> > be requeued in case of ublk_nosrv_dev_should_queue_io().
>
> In the typical case, requests issued after the ublk server dies with
> recovery enabled queue because the request_queue is quiesced. This is
> the same way things worked before this patch. There is an edge case in
> ublk_dispatch_req where we will manually requeue any request that made
> it to task_work but I assume that is not what you are talking about
> here.
>
> The above test you are highlighting looks like dead code anyways. We
> already test ubq->canceling and fail the I/O if it is set in
> ublk_prep_req, so we cannot possibly enter the if statement above.

oops, it looks one bug introduced recently by commit
("d796cea7b9f3 ublk: implement ->queue_rqs()"), and it breaks
UBLK_F_USER_RECOVERY_REISSUE.

>
> >
> > And that should be the reason why 'make test T=generic/004' hangs forever
> > after applying this patch.
>
> I am not able to reproduce the hang even after increasing LOOPS to 100
> for more repetitions. Can you ensure you have the most recent version of
> the patch? I fixed a similar hang between v2-v3.

Yeah, I confirmed it is triggered 100% on V3 by running stress_05 or ublksrv
'make test T=generic/004', please see the following stack trace:

[root@ktest-40 ~]# cat /proc/1505/stack
[<0>] msleep+0x31/0x50
[<0>] __ublk_quiesce_dev+0x50/0xa0 [ublk_drv]
[<0>] ublk_stop_dev_unlocked+0xd1/0xe0 [ublk_drv]
[<0>] ublk_stop_dev+0x29/0xf0 [ublk_drv]
[<0>] ublk_ctrl_uring_cmd+0x4c4/0x1150 [ublk_drv]
[<0>] io_uring_cmd+0x98/0x110
[<0>] io_issue_sqe+0x245/0x5a0
[<0>] io_wq_submit_work+0xc4/0x330
[<0>] io_worker_handle_work+0x143/0x570
[<0>] io_wq_worker+0xde/0x350
[<0>] ret_from_fork+0x31/0x50
[<0>] ret_from_fork_asm+0x1a/0x30

>
> >
> > > ublk_queue_cmd(ubq, rq);
> > > return BLK_STS_OK;
> > > }
> > > @@ -1461,8 +1424,71 @@ static int ublk_ch_open(struct inode *inode, struct file *filp)
> > > static int ublk_ch_release(struct inode *inode, struct file *filp)
> > > {
> > > struct ublk_device *ub = filp->private_data;
> > > + int i;
> > > +
> > > + mutex_lock(&ub->mutex);
> > > + /*
> > > + * If the device is not live, we will not transition to a nosrv
> > > + * state. This protects against:
> > > + * - accidental poking of the ublk character device
> > > + * - some ublk servers which may open/close the ublk character
> > > + * device during startup
> > > + */
> > > + if (ub->dev_info.state != UBLK_S_DEV_LIVE)
> > > + goto out;
> > > +
> > > + /*
> > > + * Since we are releasing the ublk character file descriptor, we
> > > + * know that there cannot be any concurrent file-related
> > > + * activity (e.g. uring_cmds or reads/writes). However, I/O
> > > + * might still be getting dispatched. Quiesce that too so that
> > > + * we don't need to worry about anything concurrent.
> > > + *
> > > + * We may have already quiesced the queue if we canceled any
> > > + * uring_cmds, so only quiesce if necessary (quiesce is not
> > > + * idempotent, it has an internal counter which we need to
> > > + * manage carefully).
> > > + */
> > > + if (!blk_queue_quiesced(ub->ub_disk->queue))
> > > + blk_mq_quiesce_queue(ub->ub_disk->queue);
> > > +
> > > + /*
> > > + * Handle any requests outstanding to the ublk server
> > > + */
> > > + for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> > > + ublk_abort_queue(ub, ublk_get_queue(ub, i));
> > >
> > > + /*
> > > + * Transition the device to the nosrv state. What exactly this
> > > + * means depends on the recovery flags
> > > + */
> > > + if (ublk_nosrv_should_stop_dev(ub)) {
> > > + /*
> > > + * Allow any pending/future I/O to pass through quickly
> > > + * with an error. This is needed because del_gendisk
> > > + * waits for all pending I/O to complete
> > > + */
> > > + for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> > > + ublk_get_queue(ub, i)->force_abort = true;
> > > + blk_mq_unquiesce_queue(ub->ub_disk->queue);
> > > +
> > > + ublk_stop_dev_unlocked(ub);
> > > + } else {
> > > + if (ublk_nosrv_dev_should_queue_io(ub)) {
> > > + __ublk_quiesce_dev(ub);
> >
> > Here only inflight IOs are drained, new IO still comes after queue is
> > unquiesced, then uring_cmd UAF is triggered.
>
> No, I don't think so. There are three cases here, dependent on recovery
> flags:
>
> - no recovery flags: in this case, force_abort is set before queue is
> unquiesced, so I/Os will be errored before we try to access the
> io_uring_cmd
> - just RECOVERY: queue is not actually unquiesced here at all, because
> there are two quiesce (one above, one inside __ublk_quiesce_dev), and
> only one unquiesce
> - RECOVERY|FAIL_IO: similar to no recovery flags case, except we are
> setting fail_io instead of force_abort. The two flags are very similar
> and we should consolidate in a follow-up patch

You missed UBLK_F_USER_RECOVERY_REISSUE, and ublk_nosrv_dev_should_queue_io()
returns true, for this case, your changes is basically like:

if (ublk_nosrv_dev_should_queue_io(ub))
__ublk_quiesce_dev(ub);
blk_mq_unquiesce_queue().

__ublk_quiesce_dev() just marks the device state as QUIESCED, which can't
prevent new IO from being queued, finally the above hang in DEL_DEV command
is triggered.

Actually it is easy to fix, and I'd suggest to replace ->canceling with
->nosrv flag, which is set in ublk_ch_release(), and cleared in
ublk_ch_open(), and call __ublk_abort_rq() in ublk_queue_rq() if
->nosrv is true. Pretty readable.

Thanks,
Ming