Re: [PATCH v1 02/10] bcache: set task properly in allocator_wait()

[Hannes Reinecke <hare@suse.de>]

On 01/03/2018 03:03 PM, Coly Li wrote:
> Kernel thread routine bch_allocator_thread() references macro
> allocator_wait() to wait for a condition or quit to do_exit()
> when kthread_should_stop() is true.
> 
> Macro allocator_wait() has 2 issues in setting task state, let's
> see its code piece,
> 
> 284         while (1) {                                                   \
> 285                 set_current_state(TASK_INTERRUPTIBLE);                \
> 286                 if (cond)                                             \
> 287                         break;                                        \
> 288                                                                       \
> 289                 mutex_unlock(&(ca)->set->bucket_lock);                \
> 290                 if (kthread_should_stop())                            \
> 291                         return 0;                                     \
> 292                                                                       \
> 293                 schedule();                                           \
> 294                 mutex_lock(&(ca)->set->bucket_lock);                  \
> 295         }                                                             \
> 296         __set_current_state(TASK_RUNNING);                            \
> 
> 1) At line 285, task state is set to TASK_INTERRUPTIBLE, if at line 290
> kthread_should_stop() is true, the kernel thread will terminate and return
> to kernel/kthread.s:kthread(), then calls do_exit() with TASK_INTERRUPTIBLE
> state. This is not a suggested behavior and a warning message will be
> reported by might_sleep() in do_exit() code path: "WARNING: do not call
> blocking ops when !TASK_RUNNING; state=1 set at [xxxx]".
> 
> 2) Because task state is set to TASK_INTERRUPTIBLE at line 285, when break
> while-loop the task state has to be set back to TASK_RUNNING at line 296.
> Indeed it is unncessary, if task state is set to TASK_INTERRUPTIBLE before
> calling schedule() at line 293, we don't need to set the state back to
> TASK_RUNNING at line 296 anymore. The reason is, allocator kthread is only
> woken up by wake_up_process(), this routine makes sure the task state of
> allocator kthread will be TASK_RUNNING after it returns from schedule() at
> line 294 (see kernel/sched/core.c:try_to_wake_up() for more detailed
> information).
> 
> This patch fixes the above 2 issues by,
> 1) Setting TASK_INTERRUPTIBLE state just before calling schedule().
> 2) Then setting TASK_RUNNING at line 296 is unnecessary, remove it.
> 
> Signed-off-by: Coly Li <colyli@suse.de>
> ---
>  drivers/md/bcache/alloc.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
> index a0cc1bc6d884..48c002faf08d 100644
> --- a/drivers/md/bcache/alloc.c
> +++ b/drivers/md/bcache/alloc.c
> @@ -282,7 +282,6 @@ static void invalidate_buckets(struct cache *ca)
>  #define allocator_wait(ca, cond)					\
>  do {									\
>  	while (1) {							\
> -		set_current_state(TASK_INTERRUPTIBLE);			\
>  		if (cond)						\
>  			break;						\
>  									\
> @@ -290,10 +289,10 @@ do {									\
>  		if (kthread_should_stop())				\
>  			return 0;					\
>  									\
> +		set_current_state(TASK_INTERRUPTIBLE);			\
>  		schedule();						\
>  		mutex_lock(&(ca)->set->bucket_lock);			\
>  	}								\
> -	__set_current_state(TASK_RUNNING);				\
>  } while (0)
>  
>  static int bch_allocator_push(struct cache *ca, long bucket)
> 
Same as with the previous patch; why not simply set it to TASK_RUNNING
before 'return 0' ?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
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