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

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Message-ID: <087d25cc-97b6-7daf-3722-65fba86aaf1d@huaweicloud.com>
Date:   Thu, 12 Jan 2023 14:12:22 +0800
From:   Hou Tao <houtao@...weicloud.com>
To:     Jingbo Xu <jefflexu@...ux.alibaba.com>,
        David Howells <dhowells@...hat.com>
Cc:     linux-cachefs@...hat.com, Jeff Layton <jlayton@...nel.org>,
        linux-erofs@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        houtao1@...wei.com
Subject: Re: [PATCH v2 1/2] fscache: Use wait_on_bit() to wait for the freeing
 of relinquished volume

Hi,

On 1/12/2023 11:58 AM, Jingbo Xu wrote:
>
> On 1/12/23 12:06 AM, David Howells wrote:
>> Hou Tao <houtao@...weicloud.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 for the details annotation. It is exactly what I tried to say but failed to.
>