Re: [PATCH] xfs: check kthread_should_stop() after the setting of task state
From: Brian Foster <hidden>
Date: 2017-09-08 17:42:54
On Thu, Sep 07, 2017 at 04:04:57PM +0800, Hou Tao wrote:
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 blocked indefinitely and detects the stop bit
at the next iteration.
Signed-off-by: Hou Tao <redacted>
---I assume you've verified this against your local reproducer? Otherwise just a nit on the comment...
quoted hunk ↗ jump to hunk
fs/xfs/xfs_trans_ail.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-)diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c index 9056c0f..cd6e185 100644 --- a/fs/xfs/xfs_trans_ail.c +++ b/fs/xfs/xfs_trans_ail.c@@ -499,11 +499,22 @@ 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 blocked + * indefinitely and detects the stop bit at the next iteration. + */
I'd change the "it's not blocked indefinitely" wording to something like "it's not scheduled out indefinitely." Also, a mention that the task state sets above include a memory barrier to serialize against kthread_stop() couldn't hurt. Otherwise this looks fine to me: Reviewed-by: Brian Foster <redacted>
+ if (kthread_should_stop()) {
+ __set_current_state(TASK_RUNNING);
+ break;
+ }
spin_lock(&ailp->xa_lock);
--
2.5.0
--
To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html