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 <houtao1@xxxxxxxxxx>
> ---
I assume you've verified this against your local reproducer? Otherwise
just a nit on the comment...
> 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 <bfoster@xxxxxxxxxx>
> + 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@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
