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;
--
2.39.5 (Apple Git-154)
