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xfs: check kthread_should_stop() after the setting of task state

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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@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
This commit is contained in:
Hou Tao 2017-10-17 14:16:28 -07:00 committed by Darrick J. Wong
parent f038750165
commit 0bd89676c4
1 changed files with 18 additions and 3 deletions
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21
fs/xfs/xfs_trans_ail.c
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@ -514,11 +514,26 @@ 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);