linux-kernel - Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref

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Date:   Fri, 8 Apr 2022 12:16:49 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Muchun Song <songmuchun@...edance.com>, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, zhouchengming@...edance.com
Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put()
 completes



On 2022/4/8 12:14 PM, Qi Zheng wrote:
> 
> 
> On 2022/4/8 12:10 PM, Andrew Morton wrote:
>> On Fri, 8 Apr 2022 12:06:20 +0800 Qi Zheng 
>> <zhengqi.arch@...edance.com> wrote:
>>
>>>
>>>
>>>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>>>>>>
>>>>>> Are any users affected by this?  If so, I think a Fixes tag
>>>>>> is necessary.
>>>>>
>>>>> Looks all current users(blk_pre_runtime_suspend() and 
>>>>> set_in_sync()) are
>>>>> affected by this.
>>>>>
>>>>> I see that this patch has been merged into the mm tree, can Andrew 
>>>>> help
>>>>> me add the following Fixes tag?
>>>>
>>>> Andrew is helpful ;)
>>>>
>>>> Do you see reasons why we should backport this into -stable trees?
>>>> It's 8 years old, so my uninformed guess is "no"?
>>>
>>> Hmm, although the commit 490c79a65708 add wake_up_all(), it is no
>>> problem for the usage at that time, maybe the correct Fixes tag is the
>>> following:
>>>
>>> Fixes: 210f7cdcf088 ("percpu-refcount: support synchronous switch to
>>> atomic mode.")
>>>
>>> But in fact, there is no problem with it, but all current users expect
>>> the refcount is stable after percpu_ref_switch_to_atomic_sync() returns.
>>>
>>> I have no idea as which Fixes tag to add.
>>
>> Well the solution to that problem is to add cc:stable and let Greg
>> figure it out ;)
>>
>> The more serious question is "should we backport this".  What is the
>> end-user-visible impact of the bug?  Do our users need the fix or not?
> 
> The impact on the current user is that it is possible to miss an 
> opportunity to reach 0 due to the case B in the commit message:

There may be performance issues, but should not cause serious bugs.

> 
> /* The value of &ref is unstable! */
> percpu_ref_is_zero(&ref)
>                          (B)percpu_ref_put(ref);
> 
> Thanks,
> Qi
> 
>>
> 

-- 
Thanks,
Qi