A umount hang is possible when a race occurs between the umount
process and the xfsaild kthread. The following sequences outline
the race:
xfsaild: kthread_should_stop()
=> return false, so xfsaild continue
umount: set_bit(KTHREAD_SHOULD_STOP, &kthread->flags)
=> by kthread_stop()
umount: wake_up_process()
=> because xfsaild is still running, so 0 is returned
xfsaild: __set_current_state(TASK_INTERRUPTIBLE)
xfsaild: schedule()
=> now, xfsaild will wait indefinitely
umount: wait_for_completion()
=> and umount will hang
To fix that, we need to check kthread_should_stop() after we set
the task state, so the xfsaild will either see the stop bit and
exit or the task state is reset to runnable by wake_up_process()
such that it isn't scheduled out indefinitely and detects the stop
bit at the next iteration.
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
---
v2:
* comment updates suggested by Brain Foster
v1:
* http://www.spinics.net/lists/linux-xfs/msg10285.html
---
fs/xfs/xfs_trans_ail.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
index 9056c0f..2d77d9c 100644
--- a/fs/xfs/xfs_trans_ail.c
+++ b/fs/xfs/xfs_trans_ail.c
@@ -499,11 +499,26 @@ xfsaild(
current->flags |= PF_MEMALLOC;
set_freezable();
- while (!kthread_should_stop()) {
+ while (1) {
if (tout && tout <= 20)
- __set_current_state(TASK_KILLABLE);
+ set_current_state(TASK_KILLABLE);
else
- __set_current_state(TASK_INTERRUPTIBLE);
+ set_current_state(TASK_INTERRUPTIBLE);
+
+ /*
+ * Check kthread_should_stop() after we set the task state
+ * to guarantee that we either see the stop bit and exit or
+ * the task state is reset to runnable such that it's not
+ * scheduled out indefinitely and detects the stop bit at
+ * next iteration.
+ *
+ * A memory barrier is included in above task state set to
+ * serialize again kthread_stop().
+ */
+ if (kthread_should_stop()) {
+ __set_current_state(TASK_RUNNING);
+ break;
+ }
spin_lock(&ailp->xa_lock);
--
2.5.0
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