Re: Potential hang on ublk_ctrl_del_dev()

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jan 03, 2023 at 02:51:20PM -0700, Jens Axboe wrote:
> On 1/3/23 2:47?PM, 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.
> > 
> > What do you say? Would you agree to change the behavior to return -EBUSY?
> > 
> > Thanks,
> > Nadav
> > 
> > 
> > [  974.149938] INFO: task ublk:2250 blocked for more than 120 seconds.
> > [  974.157786]       Not tainted 6.1.0 #30
> > [  974.162369] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  974.171417] task:ublk            state:D stack:0     pid:2250  ppid:2249   flags:0x00004004
> > [  974.181054] Call Trace:
> > [  974.184097]  <TASK>
> > [  974.186726]  __schedule+0x37e/0xe10
> > [  974.190915]  ? __this_cpu_preempt_check+0x13/0x20
> > [  974.196463]  ? lock_release+0x133/0x2a0
> > [  974.201043]  schedule+0x67/0xe0
> > [  974.204846]  ublk_ctrl_uring_cmd+0xf45/0x1110
> > [  974.210016]  ? lock_is_held_type+0xdd/0x130
> > [  974.214990]  ? var_wake_function+0x60/0x60
> > [  974.219872]  ? rcu_read_lock_sched_held+0x4f/0x80
> > [  974.225443]  io_uring_cmd+0x9a/0x130
> > [  974.229743]  ? io_uring_cmd_prep+0xf0/0xf0
> > [  974.234638]  io_issue_sqe+0xfe/0x340
> > [  974.238946]  io_submit_sqes+0x231/0x750
> > [  974.243553]  __x64_sys_io_uring_enter+0x22b/0x640
> > [  974.249134]  ? trace_hardirqs_on+0x3c/0xe0
> > [  974.254042]  do_syscall_64+0x35/0x80
> > [  974.258361]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> 
> Ming, this also looks like ublk doesn't always honor
> IO_URING_F_NONBLOCK, we can't be sleeping like that under issue. Then it
> should be bounced with -EAGAIN and retried from an io-wq worker.

Yeah, you are right, and looks the following change is needed and all
ublk control commands are actually handled in sync style from userspace.

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 144eda037646..8011ae1f20d5 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -2264,6 +2264,9 @@ static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 	struct ublk_device *ub = NULL;
 	int ret = -EINVAL;
 
+	if (issue_flags & IO_URING_F_NONBLOCK)
+		return -EAGAIN;
+
 	ublk_ctrl_cmd_dump(cmd);
 
 	if (!(issue_flags & IO_URING_F_SQE128))

thanks,
Ming