On Fri, 8 Apr 2022 12:06:20 +0800 Qi Zheng <[email protected]> wrote:
>
>
> >>>> Signed-off-by: Qi Zheng <[email protected]>
> >>>
> >>> 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?
On 2022/4/8 12:10 PM, Andrew Morton wrote:
> On Fri, 8 Apr 2022 12:06:20 +0800 Qi Zheng <[email protected]> wrote:
>
>>
>>
>>>>>> Signed-off-by: Qi Zheng <[email protected]>
>>>>>
>>>>> 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
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
>> <[email protected]> wrote:
>>
>>>
>>>
>>>>>>> Signed-off-by: Qi Zheng <[email protected]>
>>>>>>
>>>>>> 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
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 <[email protected]> wrote:
> >
> > >
> > >
> > > > > > > Signed-off-by: Qi Zheng <[email protected]>
> > > > > >
> > > > > > 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?
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
faster than an atomic decrement. The q_usage_counter allows reinit so it
skips the __percpu_ref_exit() call.
Thanks,
Dennis
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 <[email protected]> wrote:
>>>
>>>>
>>>>
>>>>>>>> Signed-off-by: Qi Zheng <[email protected]>
>>>>>>>
>>>>>>> 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