We recently encountered an XFS deadlock issue, which is a known problem
resolved in the upstream kernel [0]. During the analysis of this issue, I
observed that a kernel thread in the TASK_WAKEKILL state could not be
detected as a hung task by the hung_task detector. The details are as
follows:
Using the following command, I identified nine tasks stuck in the D state:
$ ps -eLo state,comm,tid,wchan | grep ^D
D java 4177339 xfs_buf_lock
D kworker/93:3+xf 3025535 xfs_buf_lock
D kworker/87:0+xf 3426612 xfs_extent_busy_flush
D kworker/85:0+xf 3479378 xfs_buf_lock
D kworker/91:1+xf 3584478 xfs_buf_lock
D kworker/80:3+xf 3655680 xfs_buf_lock
D kworker/89:0+xf 3671691 xfs_buf_lock
D kworker/84:1+xf 3708397 xfs_buf_lock
D kworker/81:1+xf 4005763 xfs_buf_lock
However, the hung_task detector only reported eight of these tasks:
[3108840.650652] INFO: task java:4177339 blocked for more than 247779 seconds.
[3108840.654197] INFO: task kworker/93:3:3025535 blocked for more than 248427 seconds.
[3108840.657711] INFO: task kworker/85:0:3479378 blocked for more than 247836 seconds.
[3108840.661483] INFO: task kworker/91:1:3584478 blocked for more than 249638 seconds.
[3108840.664871] INFO: task kworker/80:3:3655680 blocked for more than 249638 seconds.
[3108840.668495] INFO: task kworker/89:0:3671691 blocked for more than 249047 seconds.
[3108840.672418] INFO: task kworker/84:1:3708397 blocked for more than 247836 seconds.
[3108840.676175] INFO: task kworker/81:1:4005763 blocked for more than 247836 seconds.
Task 3426612, although in the D state, was not reported as a hung task.
I confirmed that task 3426612 remained in the D (disk sleep) state and
experienced no context switches over a long period:
$ cat /proc/3426612/status | grep -E "State:|ctxt_switches:"; \
sleep 60; echo "----"; \
cat /proc/3426612/status | grep -E "State:|ctxt_switches:"
State: D (disk sleep)
voluntary_ctxt_switches: 7516
nonvoluntary_ctxt_switches: 0
----
State: D (disk sleep)
voluntary_ctxt_switches: 7516
nonvoluntary_ctxt_switches: 0
The system's hung_task detector settings were configured as follows:
kernel.hung_task_timeout_secs = 28
kernel.hung_task_warnings = -1
The issue lies in the handling of task state in the XFS code. Specifically,
the thread in question (3426612) was set to the TASK_KILLABLE state in
xfs_extent_busy_flush():
xfs_extent_busy_flush
prepare_to_wait(&pag->pagb_wait, &wait, TASK_KILLABLE);
When a task is in the TASK_WAKEKILL state (a subset of TASK_KILLABLE), the
hung_task detector ignores it, as it assumes such tasks can be terminated.
However, in this case, the kernel thread cannot be killed, meaning it
effectively becomes a hung task.
To address this issue, the hung_task detector should report the kthreads in
the TASK_WAKEKILL state.
Link: https://lore.kernel.org/linux-xfs/20230620002021.1038067-5-david@xxxxxxxxxxxxx/ [0]
Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
---
kernel/hung_task.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index c18717189f32..ed63fd84ce2e 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -220,8 +220,9 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
*/
state = READ_ONCE(t->__state);
if ((state & TASK_UNINTERRUPTIBLE) &&
+ (t->flags & PF_KTHREAD ||
!(state & TASK_WAKEKILL) &&
- !(state & TASK_NOLOAD))
+ !(state & TASK_NOLOAD)))
check_hung_task(t, timeout);
}
unlock:
--
2.43.5
