[PATCH V2 2/2] md/raid10: fix missing wakeup in wait_barrier_nolock

[BUG]
Recently, our fuzz testing triggered a hungtask issue in RAID10:

INFO: task md0_raid10:1273 blocked for more than 120 seconds.
      Not tainted 7.2.0-rc6+ #94
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:md0_raid10      state:D stack:0     pid:1273  tgid:1273  ppid:2
Call Trace:
 <TASK>
 __schedule+0xdf9/0x5c90
 ? _raw_spin_unlock_irqrestore+0xe/0x40
 schedule+0x74/0x1f0
 raid10d.cold+0x7db/0x1992
 md_thread+0x1ce/0x3e0
 kthread+0x327/0x410
 ......

[CAUSE]
The root cause of the issue is as follows:
[read process1]			[read process2]			[raid10d]
raid10_make_request
...
 // nr_pending == 1
 atomic_inc(&conf->nr_pending)
...
raid10_end_read_request
 reschedule_retry
  md_wakeup_thread(mddev->thread)
				raid10_read_request
				 regular_request_wait
				  wait_barrier
				   wait_barrier_nolock
				    seq = read_seqbegin(&conf->resync_lock)
				    // nr_pending == 2
				    atomic_inc(&conf->nr_pending)
								raid10d
								 handle_read_error
								  freeze_array
								   write_seqlock_irq(&conf->resync_lock)
								   conf->array_freeze_pending++
								   WRITE_ONCE(conf->barrier, conf->barrier + 1)
								   conf->nr_waiting++
								   // nr_pending == 2, nr_queued == 0
								   wait_event_barrier_cmd
								    write_sequnlock_irq(&(conf)->resync_lock)
								    schedule	// hungtask!!
				    read_seqretry(&conf->resync_lock, seq)
				    // not wakeup because conf->nr_pending == 1
				    atomic_dec_and_test(&conf->nr_pending)
				   /* Hungtask will also occur here: since the barrier is non-zero,
				    * this I/O can never complete, so it can't call allow_barrier()
				    * to wake up the pending freeze_array(). */
				   wait_event_barrier(conf, stop_waiting_barrier(conf))

wait_barrier_nolock() speculatively increments nr_pending and, on a seqlock
retry, rolls it back with atomic_dec_and_test(). The wake fires only when
nr_pending reaches 0. This causes the freeze_array() in the aforementioned
raid10d flow to never be woken up.

[FIX]
Referring to allow_barrier(), this issue can be fixed by adding a wake-up
condition for "conf->array_freeze_pending" in wait_barrier_nolock().

Fixes: b9b083f9044a ("md/raid10: convert resync_lock to use seqlock")
Signed-off-by: Zizhi Wo <redacted>
---
 drivers/md/raid10.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index dc40110a6736..8dadf13c2b4d 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c

@@ -994,11 +994,12 @@ static bool wait_barrier_nolock(struct r10conf *conf)
 
 	atomic_inc(&conf->nr_pending);
 	if (!read_seqretry(&conf->resync_lock, seq))
 		return true;
 
-	if (atomic_dec_and_test(&conf->nr_pending))
+	if (atomic_dec_and_test(&conf->nr_pending) ||
+	    READ_ONCE(conf->array_freeze_pending))
 		wake_up_barrier(conf);
 
 	return false;
 }
 

-- 
2.52.0