Re: [PATCH] xfs: check kthread_should_stop() after the setting of task state

[Brian Foster <bfoster@redhat.com>]

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@huawei.com>
> ---

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@redhat.com>

> +		if (kthread_should_stop()) {
> +			__set_current_state(TASK_RUNNING);
> +			break;
> +		}
>  
>  		spin_lock(&ailp->xa_lock);
>  
> -- 
> 2.5.0
> 
> --
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