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Message-ID: <aad36908-3e95-a8c9-533d-2a39ea76cd3a@bytedance.com>
Date:   Fri, 8 Apr 2022 14:28:41 +0800
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Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put()
 completes
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To:     Dennis Zhou <dennis@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Muchun Song <songmuchun@bytedance.com>, 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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 <20220407211018.875696691e4411a7b5c8f63f@linux-foundation.org>
 <d546cbce-b1d6-9499-3093-796cad7be9aa@bytedance.com>
 <Yk/OtIKckFprZrGx@fedora>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <Yk/OtIKckFprZrGx@fedora>
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On 2022/4/8 1:57 PM, Dennis Zhou wrote:
> On Fri, Apr 08, 2022 at 12:14:54PM +0800, 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@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:
>>
> 
> Did you find this bug through code inspection or was the finding
> motivated by a production incident?

I find this bug through code inspection, because I want to use
percpu_ref_switch_to_atomic_sync()+percpu_ref_is_zero() to do something
similar.

> 
> The usage in block/blk-pm.c looks problematic, but I'm guessing this is
> a really, really hard bug to trigger. You need to have the wake up be

Agree, I manually added the delay in wake_up_all() and percpu_ref_put()
to trigger the case B.

> faster than an atomic decrement. The q_usage_counter allows reinit so it
> skips the __percpu_ref_exit() call.
> 
> Thanks,
> Dennis

-- 
Thanks,
Qi