Re: [PATCH] bcache: set task state correctly in allocator_wait()

[Hannes Reinecke <hare@suse.de>]

On 11/22/2017 01:33 PM, Coly Li wrote:
> Kthread function bch_allocator_thread() references allocator_wait(ca, cond)
> and when kthread_should_stop() is true, this kthread exits.
> 
> The problem is, if kthread_should_stop() is true, macro allocator_wait()
> calls "return 0" with current task state TASK_INTERRUPTIBLE. After function
> bch_allocator_thread() returns to do_exit(), there are some blocking
> operations are called, then a kenrel warning is popped up by __might_sleep
> from kernel/sched/core.c,
>   "WARNING: do not call blocking ops when !TASK_RUNNING; state=1 set at [xxxx]"
> 
> If the task is interrupted and preempted out, since its status is
> TASK_INTERRUPTIBLE, it means scheduler won't pick it back to run forever,
> and the allocator thread may hang in do_exit().
> 
> This patch sets allocator kthread state back to TASK_RUNNING before it
> returns to do_exit(), which avoids a potential deadlock.
> 
> Signed-off-by: Coly Li <colyli@suse.de>
> Cc: stable@vger.kernel.org
> ---
>  drivers/md/bcache/alloc.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
> index a27d85232ce1..996ebbabd819 100644
> --- a/drivers/md/bcache/alloc.c
> +++ b/drivers/md/bcache/alloc.c
> @@ -286,9 +286,12 @@ do {									\
>  		if (cond)						\
>  			break;						\
>  									\
> +									\
>  		mutex_unlock(&(ca)->set->bucket_lock);			\
> -		if (kthread_should_stop())				\
> +		if (kthread_should_stop()) {				\
> +			__set_current_state(TASK_RUNNING);		\
>  			return 0;					\
> +		}							\
>  									\
>  		schedule();						\
>  		mutex_lock(&(ca)->set->bucket_lock);			\
> 
_Actually_ there is a push to remove all kthreads in the kernel, as they
don't play nice together with RT.
So while you're at it, do you think it'd be possible to convert it to a
workqueue? Sebastian will be happy to help you here, right, Sebastian?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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