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Message-ID: <d546cbce-b1d6-9499-3093-796cad7be9aa@bytedance.com>
Date:   Fri, 8 Apr 2022 12:14:54 +0800
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Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put()
 completes
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To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Muchun Song <songmuchun@bytedance.com>, dennis@kernel.org,
        tj@kernel.org, cl@linux.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, zhouchengming@bytedance.com
References: <20220407103335.36885-1-zhengqi.arch@bytedance.com>
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 <35195a61-d531-aeb2-5565-146e345f8bf6@bytedance.com>
 <20220407211018.875696691e4411a7b5c8f63f@linux-foundation.org>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <20220407211018.875696691e4411a7b5c8f63f@linux-foundation.org>
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On 2022/4/8 12:10 PM, Andrew Morton wrote:
> On Fri, 8 Apr 2022 12:06:20 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> 
>>
>>
>>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.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:

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

Thanks,
Qi

> 

-- 
Thanks,
Qi