On Thu, 28 May 2026, Fengnan wrote:
> ping..
Hi
I already staged this patch in the linux-dm repository.
Mikulas
> 在 2026/5/13 17:13, Fengnan Chang 写道:
> > dm_poll_bio() is the ->poll_bio() callback for a stacked dm device.
> > The caller only knows about the dm queue, so it may decide to do a
> > spinning poll if it thinks a single queue is being polled. Passing those
> > flags unchanged to the mapped clone lets blk_mq_poll() spin on a target
> > queue from inside dm_poll_bio().
> >
> > With io_uring IOPOLL on a dm-stripe target this can keep a task in
> >
> > dm_poll_bio() -> bio_poll() -> blk_mq_poll()
> >
> > long enough to trigger an RCU CPU stall, before io_uring gets back to
> > io_iopoll_check() and its need_resched() check.
> >
> > Keep dm's ->poll_bio() bounded by forcing one-shot polling for target
> > bios. The caller can invoke dm_poll_bio() again if it wants to keep
> > polling, and it also gets a chance to reap completions or reschedule
> > between passes.
> >
> > Fixes: f22ecf9c14c1 ("blk-mq: delete task running check in blk_hctx_poll()")
> > Signed-off-by: Fengnan Chang <[email protected]>
> > ---
> > drivers/md/dm.c | 13 +++++++++++--
> > 1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index e178fe19973ea..8f44fbbcf3da2 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -2098,8 +2098,17 @@ static bool dm_poll_dm_io(struct dm_io *io, struct
> > io_comp_batch *iob,
> > WARN_ON_ONCE(!dm_tio_is_normal(&io->tio));
> >
> > /* don't poll if the mapped io is done */
> > - if (atomic_read(&io->io_count) > 1)
> > - bio_poll(&io->tio.clone, iob, flags);
> > + if (atomic_read(&io->io_count) > 1) {
> > + /*
> > + * DM hides the target queues from the upper poller, which may
> > + * decide it is safe to spin on a single stacked queue. Do not
> > + * pass that spinning policy down to a target queue: one slow
> > + * clone could keep the task inside dm_poll_bio() for a long
> > + * time. Poll target bios once and let the caller decide
> > + * whether to keep polling, reap completions or reschedule.
> > + */
> > + bio_poll(&io->tio.clone, iob, flags | BLK_POLL_ONESHOT);
> > + }
> >
> > /* bio_poll holds the last reference */
> > return atomic_read(&io->io_count) == 1;
>
