Re: [PATCH v2 1/2] fscache: Use wait_on_bit() to wait for the freeing of relinquished volume

[Jingbo Xu <jefflexu@linux.alibaba.com>]

On 1/12/23 12:06 AM, David Howells wrote:
> Hou Tao <houtao@huaweicloud.com> wrote:
> 
>>  			clear_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &cursor->flags);
>> +			/*
>> +			 * Paired with barrier in wait_on_bit(). Check
>> +			 * wake_up_bit() and waitqueue_active() for details.
>> +			 */
>> +			smp_mb__after_atomic();
>>  			wake_up_bit(&cursor->flags, FSCACHE_VOLUME_ACQUIRE_PENDING);
> 
> What two values are you applying a partial ordering to?

Yeah Hou Tao has explained that a full barrier is needed here to avoid
the potential reordering at the waker side.

As I was also researching on this these days, I'd like to share my
thought on this, hopefully if it could give some insight :)

Without the barrier at the waker side, it may suffer from the following
race:

```
CPU0 - waker                    CPU1 - waiter

if (waitqueue_active(wq_head)) <-- find no wq_entry in wq_head list
    wake_up(wq_head);

                                for (;;) {
                                   prepare_to_wait(...);
                                        # add wq_entry into wq_head list

                                    if (@cond)  <-- @cond is false
                                        break;
                                    schedule(); <-- wq_entry still in
                                                    wq_head list,
                                                    wait for next wakeup
                                 }
                                 finish_wait(&wq_head, &wait);

@cond = true;
```

in which case the waiter misses the wakeup for one time.

-- 
Thanks,
Jingbo