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

Coly Li
2018-01-05T17:11:44Z

On 04/01/2018 1:09 AM, Michael Lyle wrote:
> On 01/03/2018 06:03 AM, 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>

Hi Mike,

I also feel I made mistake here. set_current_state(TASK_INTERRUPTIBLE)
must be called before the condition check, other wise there will be a
race window on task state setting. A correct fix should be setting task
state to TASK_RUNNING before calling 'return 0', or before calling
mutex_unlock(&(ca)->set->bucket_lock).

When I review the new closure_sync() patches, I feel I make mistake
here. It will be fixed in v2 patch set.

Thanks.

Coly Li