Prev: [PATCH 1/4] perf ui: Make SPACE work as PGDN in all browsers
Next: [PATCH 4/4] perf ui hist browser: Fixup key bindings
From: Tomasz Buchert on 11 Aug 2010 03:40
Matt Helsley a �crit :
> On Tue, Aug 10, 2010 at 09:53:21PM +0200, Tomasz Buchert wrote:
>> Writing 'FROZEN' to freezer.state file does not
>> forbid the task to be moved away from its cgroup
>> (for a very short time). Nevertheless the moved task
>> can become frozen OUTSIDE its cgroup which puts
>> discussed task in a permanent 'D' state.
>
> SUMMARY (for supporting info see the "DETAILS" heading below)
>
> I can't reproduce this.
>
> My preliminary conclusion is that your testcase doesn't really reproduce
> what you described. Instead, your testcase prints an incorrect message which
> could easily lead the person running it to the wrong conclusion.
>
> DETAILS
>
> I tried it with and without the cpuset portions of the testcase on a dual
> core bare metal system with a 2.6.31-derived distro kernel. Since it
> *should* be obeying the cpuset portions of the testcase I don't think the
> fact that it's dual-core should make a difference.
>
> I also tried with a 2.6.34-rc5 kernel in a single-cpu kvm on a different
> distro (but on the same physical hardware). I also tried it with and without
> the sleep(1) in the child's for(;;) loop in case the cpu load somehow enabled
> me to trigger the race.
>
> I used the following bash snippet to run the testcase variations and did not
> observe any tasks in the D state:
>
> mount -t cgroup -o freezer,cpuset none /cg
> while /bin/true ; do
> ./bug /cg
> ps -C bug -o state= | grep D && break
> done
>
> If there is a race then I should be able to run that ps line anytime afterwards
> to see the stuck task.
>
> Note that the test message:
>
> printf("Succesfully moved frozen task!\n");
>
> is bogus. In fact there is no guarantee the task or cgroup is frozen at that
> point in the testcase. This should be apparent from a careful reading of
> Documentation/cgroups/freezer-subsystem.txt, especially:
>
> This is the basic mechanism which should do the right thing for user space task
> in a simple scenario.
>
> It's important to note that freezing can be incomplete. In that case we return
> EBUSY. This means that some tasks in the cgroup are busy doing something that
> prevents us from completely freezing the cgroup at this time. After EBUSY,
> the cgroup will remain partially frozen -- reflected by freezer.state reporting
> "FREEZING" when read. The state will remain "FREEZING" until one of these
> things happens:
>
> 1) Userspace cancels the freezing operation by writing "THAWED" to
> the freezer.state file
> 2) Userspace retries the freezing operation by writing "FROZEN" to
> the freezer.state file (writing "FREEZING" is not legal
> and returns EINVAL)
> 3) The tasks that blocked the cgroup from entering the "FROZEN"
> state disappear from the cgroup's set of tasks.
>
> So simply writing FROZEN to freezer.state is necessary to initiate freezing
> but insufficient to assert that the task and/or cgroup is frozen.
> That's why the FREEZING state exists. It's intentionally not specified when/why
> we can't immediately enter FROZEN. Thus userspace must read the freezer.state
> to determine if the current state matches the requested/expected state.
>
> This is why I have the extra ps step in the script above -- to determine
> if the task is actually in D. I should also check that the cgroup it belongs
> to is THAWED. However while attempting to reproduce your report that hasn't
> been necessary -- none of the tasks have even entered the D state.
>
> Which brings us to the final portion of this analysis: Why isn't anything
> entering the D state?
>
> The behavior I have been able to reproduce and which is not a bug is
> moving the task immediately after writing FROZEN to freezer.state. We don't
> know the state of the task or cgroup at that time (in this testcase) so
> this is acceptable. I've even made a sequence of modifications to your
> testcase and run it after each modification to bring it successively more
> in line with correct use of the cgroup freezer. I still was unable to
> reproduce your report.
>
> So I'm fairly confident there is no bug. I say "fairly" because there may
> be some aspect of your system that I am not reproducing. At this point it
> would be great if you could provide more details so I can more thoroughly
> attempt to recreate your conditions.
>
> Cheers,
> -Matt Helsley
>

Maybe this is a problem with different timings. I have a qemu minimal image
with two different kernels 2.6.35 - vanilla and patached. I don't use kvm
with qemu though. You can get it from here:
http://pentium.hopto.org/~thinred/files/qemu.tar.bz2
in the package there is also minimal '.config' for current Linus's tree.
You run it with ./run-linux <bzImageOfTheKernel>. In /root you will have
'minimal' program which is my testcase. The small problem with this image is that
ps segfaults but it's not a big problem. :)

I am also aware that:
printf("Succesfully moved frozen task!\n");
does not show that task was moved. Only the message is just wrong. What I wanted to observe
is that the kernel actually ALLOWED to start the process of freezing.
When you run the testcase with 'strace' you'll see that 'write' returns a positive number
instead of -EBUSY. And that's the bug.

Anyway, I deduce from your later mail that you actually reproduced the problem.

Cheers,
Tomasz

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Tomasz Buchert on 11 Aug 2010 04:10
Tomasz Buchert a �crit :
> Matt Helsley a �crit :
>> On Tue, Aug 10, 2010 at 09:53:21PM +0200, Tomasz Buchert wrote:
>>> Writing 'FROZEN' to freezer.state file does not
>>> forbid the task to be moved away from its cgroup
>>> (for a very short time). Nevertheless the moved task
>>> can become frozen OUTSIDE its cgroup which puts
>>> discussed task in a permanent 'D' state.
>> SUMMARY (for supporting info see the "DETAILS" heading below)
>>
>> I can't reproduce this.
>>
>> My preliminary conclusion is that your testcase doesn't really reproduce
>> what you described. Instead, your testcase prints an incorrect message which
>> could easily lead the person running it to the wrong conclusion.
>>
>> DETAILS
>>
>> I tried it with and without the cpuset portions of the testcase on a dual
>> core bare metal system with a 2.6.31-derived distro kernel. Since it
>> *should* be obeying the cpuset portions of the testcase I don't think the
>> fact that it's dual-core should make a difference.
>>
>> I also tried with a 2.6.34-rc5 kernel in a single-cpu kvm on a different
>> distro (but on the same physical hardware). I also tried it with and without
>> the sleep(1) in the child's for(;;) loop in case the cpu load somehow enabled
>> me to trigger the race.
>>
>> I used the following bash snippet to run the testcase variations and did not
>> observe any tasks in the D state:
>>
>> mount -t cgroup -o freezer,cpuset none /cg
>> while /bin/true ; do
>> ./bug /cg
>> ps -C bug -o state= | grep D && break
>> done
>>
>> If there is a race then I should be able to run that ps line anytime afterwards
>> to see the stuck task.
>>
>> Note that the test message:
>>
>> printf("Succesfully moved frozen task!\n");
>>
>> is bogus. In fact there is no guarantee the task or cgroup is frozen at that
>> point in the testcase. This should be apparent from a careful reading of
>> Documentation/cgroups/freezer-subsystem.txt, especially:
>>
>> This is the basic mechanism which should do the right thing for user space task
>> in a simple scenario.
>>
>> It's important to note that freezing can be incomplete. In that case we return
>> EBUSY. This means that some tasks in the cgroup are busy doing something that
>> prevents us from completely freezing the cgroup at this time. After EBUSY,
>> the cgroup will remain partially frozen -- reflected by freezer.state reporting
>> "FREEZING" when read. The state will remain "FREEZING" until one of these
>> things happens:
>>
>> 1) Userspace cancels the freezing operation by writing "THAWED" to
>> the freezer.state file
>> 2) Userspace retries the freezing operation by writing "FROZEN" to
>> the freezer.state file (writing "FREEZING" is not legal
>> and returns EINVAL)
>> 3) The tasks that blocked the cgroup from entering the "FROZEN"
>> state disappear from the cgroup's set of tasks.
>>
>> So simply writing FROZEN to freezer.state is necessary to initiate freezing
>> but insufficient to assert that the task and/or cgroup is frozen.
>> That's why the FREEZING state exists. It's intentionally not specified when/why
>> we can't immediately enter FROZEN. Thus userspace must read the freezer.state
>> to determine if the current state matches the requested/expected state.
>>
>> This is why I have the extra ps step in the script above -- to determine
>> if the task is actually in D. I should also check that the cgroup it belongs
>> to is THAWED. However while attempting to reproduce your report that hasn't
>> been necessary -- none of the tasks have even entered the D state.
>>
>> Which brings us to the final portion of this analysis: Why isn't anything
>> entering the D state?
>>
>> The behavior I have been able to reproduce and which is not a bug is
>> moving the task immediately after writing FROZEN to freezer.state. We don't
>> know the state of the task or cgroup at that time (in this testcase) so
>> this is acceptable. I've even made a sequence of modifications to your
>> testcase and run it after each modification to bring it successively more
>> in line with correct use of the cgroup freezer. I still was unable to
>> reproduce your report.
>>
>> So I'm fairly confident there is no bug. I say "fairly" because there may
>> be some aspect of your system that I am not reproducing. At this point it
>> would be great if you could provide more details so I can more thoroughly
>> attempt to recreate your conditions.
>>
>> Cheers,
>> -Matt Helsley
>>
>
> Maybe this is a problem with different timings. I have a qemu minimal image
> with two different kernels 2.6.35 - vanilla and patached. I don't use kvm
> with qemu though. You can get it from here:
> http://pentium.hopto.org/~thinred/files/qemu.tar.bz2
> in the package there is also minimal '.config' for current Linus's tree.
> You run it with ./run-linux <bzImageOfTheKernel>. In /root you will have
> 'minimal' program which is my testcase. The small problem with this image is that
> ps segfaults but it's not a big problem. :)
>
> I am also aware that:
> printf("Succesfully moved frozen task!\n");
> does not show that task was moved. Only the message is just wrong. What I wanted to observe
> is that the kernel actually ALLOWED to start the process of freezing.
> When you run the testcase with 'strace' you'll see that 'write' returns a positive number
> instead of -EBUSY. And that's the bug.
>
> Anyway, I deduce from your later mail that you actually reproduced the problem.
>
> Cheers,
> Tomasz
>
>
The penultimate paragraph should be:

I am also aware that:
printf("Succesfully moved frozen task!\n");
does not show that task was moved. Only the message is just wrong. What I wanted to observe
is that the kernel actually ALLOWED to start the process of **MOVING**.
When you run the testcase with 'strace' you'll see that 'write' returns a positive number
instead of -EBUSY. And that's the bug.

Cheers again!
Tomasz
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Matt Helsley on 11 Aug 2010 20:30
On Wed, Aug 11, 2010 at 09:35:43AM +0200, Tomasz Buchert wrote:
> Matt Helsley a �crit :
> > On Wed, Aug 11, 2010 at 12:18:44AM +0200, Tomasz Buchert wrote:
> >> Matt Helsley a �crit :
> >>> On Tue, Aug 10, 2010 at 09:53:21PM +0200, Tomasz Buchert wrote:
> >>>> Writing 'FROZEN' to freezer.state file does not
> >>>> forbid the task to be moved away from its cgroup
> >>>> (for a very short time). Nevertheless the moved task
> >>>> can become frozen OUTSIDE its cgroup which puts
> >>>> discussed task in a permanent 'D' state.
> >>>>
> >>>> This patch forbids migration of either FROZEN
> >>>> or FREEZING tasks.
> >>>>
> >>>> This behavior was observed and easily reproduced on
> >>>> a single core laptop. Program and instructions how
> >>>> to reproduce the bug can be fetched from:
> >>>> http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug.c
> >>> Thanks for the report and the test code.
> >>>
> >>> I'm will try to reproduce this race in the next few hours and analyze
> >>> it since I'm not sure the patch really fixes the race -- it may only
> >>> make the race trigger less frequently.
> >>>
> >>> At the very least the patch won't break the current code since it's
> >>> essentially a more-strict version of is_task_frozen_enough() -- it lets
> >>> fewer tasks attach/detach to/from frozen cgroups.
> >>>
> >>> Cheers,
> >>> -Matt Helsley
> >> Hi Matt!
> >> I am a novice if it comes to the kernel and I find the cgroup_freezer
> >> code especially complicated, so definetely this may be not enough to fix that.
> >> Notice also that if you uncomment the line 55 in my testcase this will also
> >> trigger the race! This, however, makes sense since process may not be in the cgroup anymore
> >> and consequently won't be thawed.
> >
> > OK, I triggered it with that. Interesting.
> >
>
> Good!
>
> >> I think that this patch fixes these problems because it does the flag checking in a right order:
> >> first freezing() is used and then frozen() which assures (see frozen_process()) that
> >> the race will not happen. Right? :)
> >
> > I see what you mean. It still seems like it wouldn't actually fix the race -- just make it
> > harder to trigger. I think you're saying this is what happens without the patch:
> >
> > Time "bug" goes through these states cgroup code checks for these states
> > -----------------------------------------------------------------------------------
> > | freezing
> > | is_frozen? Nope.
> > | frozen
> > | is_freezing? Nope.
> > | <move>
> > V
> >
> My first scenario was a bit different:
> Time "bug" goes through these states cgroup code checks for these states
> -----------------------------------------------------------------------------------
> | freezing
> | is_task_frozen_enough? Nope.
> | <move>
> | frozen
> V
> but the problem is the same.

I think I found a bug in the cgroup freezer which your patch fixes.
However I don't think it's a race.

/* Task is frozen or will freeze immediately when next it gets woken */
static bool is_task_frozen_enough(struct task_struct *task)
{
return frozen(task) ||
(task_is_stopped_or_traced(task) && freezing(task));
}

So it will only refuse to attach freezing tasks which have been stopped
or traced! Yet for attach we need to refuse to move _any_ freezing tasks.

Though stopped/traced _is_ relevant to the cgroup freezer state itself.
If we uses frozen(task) || freezing(task) to determine whether a cgroup
is frozen then it would be possible for the task to still be active
when the cgroup is finally reported FROZEN. However that's not possible
when the task is stopped/traced *and* freezing since even if woken it
won't exit the kernel before entering the refrigerator.

> > But, without having carefully investigated the details, this could just as easily happen
> > with your patch:
> >
> > Time "bug" goes through these states cgroup code checks for these states
> > -----------------------------------------------------------------------------------
> > | is_freezing? Nope.
> > | is_frozen? Nope.
> > | freezing
> > | <move>
> > | frozen
> > V
> >
>
> This can't happen as far as I know because there is cgroup_lock around the code in freezer_write()
> and freezer_can_attach().
> The task can't enter 'freezing' state when can_attach is executed.

You're right, cgroup_mutex should protect against that. However presumably
that same logic applies to the first case. So again I don't think the bug
is a race.

<snipped the remaining cases which should be the same>

At this point I think the bug description in your patch needs to change
but the patch itself is perfect. Assuming you agree with my assessment
of the bug I think the process is: you'll rewrite the description, add -stable
maintaners to your current Cc's (since this bug is simple, exists in previous
versions, and is somewhat nasty), add:

Reviewed-by: Matt Helsley <matthltc(a)us.ibm.com>
Tested-by: Matt Helsley <matthltc(a)us.ibm.com>

and send it to Andrew Morton. Hopefully then (if not before) Paul and Li
will ack it.

Thanks!

Cheers,
-Matt Helsley
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Tomasz Buchert on 11 Aug 2010 21:00
Matt Helsley a �crit :
> On Wed, Aug 11, 2010 at 09:35:43AM +0200, Tomasz Buchert wrote:
>> Matt Helsley a �crit :
>>> On Wed, Aug 11, 2010 at 12:18:44AM +0200, Tomasz Buchert wrote:
>>>> Matt Helsley a �crit :
>>>>> On Tue, Aug 10, 2010 at 09:53:21PM +0200, Tomasz Buchert wrote:
>>>>>> Writing 'FROZEN' to freezer.state file does not
>>>>>> forbid the task to be moved away from its cgroup
>>>>>> (for a very short time). Nevertheless the moved task
>>>>>> can become frozen OUTSIDE its cgroup which puts
>>>>>> discussed task in a permanent 'D' state.
>>>>>>
>>>>>> This patch forbids migration of either FROZEN
>>>>>> or FREEZING tasks.
>>>>>>
>>>>>> This behavior was observed and easily reproduced on
>>>>>> a single core laptop. Program and instructions how
>>>>>> to reproduce the bug can be fetched from:
>>>>>> http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug.c
>>>>> Thanks for the report and the test code.
>>>>>
>>>>> I'm will try to reproduce this race in the next few hours and analyze
>>>>> it since I'm not sure the patch really fixes the race -- it may only
>>>>> make the race trigger less frequently.
>>>>>
>>>>> At the very least the patch won't break the current code since it's
>>>>> essentially a more-strict version of is_task_frozen_enough() -- it lets
>>>>> fewer tasks attach/detach to/from frozen cgroups.
>>>>>
>>>>> Cheers,
>>>>> -Matt Helsley
>>>> Hi Matt!
>>>> I am a novice if it comes to the kernel and I find the cgroup_freezer
>>>> code especially complicated, so definetely this may be not enough to fix that.
>>>> Notice also that if you uncomment the line 55 in my testcase this will also
>>>> trigger the race! This, however, makes sense since process may not be in the cgroup anymore
>>>> and consequently won't be thawed.
>>> OK, I triggered it with that. Interesting.
>>>
>> Good!
>>
>>>> I think that this patch fixes these problems because it does the flag checking in a right order:
>>>> first freezing() is used and then frozen() which assures (see frozen_process()) that
>>>> the race will not happen. Right? :)
>>> I see what you mean. It still seems like it wouldn't actually fix the race -- just make it
>>> harder to trigger. I think you're saying this is what happens without the patch:
>>>
>>> Time "bug" goes through these states cgroup code checks for these states
>>> -----------------------------------------------------------------------------------
>>> | freezing
>>> | is_frozen? Nope.
>>> | frozen
>>> | is_freezing? Nope.
>>> | <move>
>>> V
>>>
>> My first scenario was a bit different:
>> Time "bug" goes through these states cgroup code checks for these states
>> -----------------------------------------------------------------------------------
>> | freezing
>> | is_task_frozen_enough? Nope.
>> | <move>
>> | frozen
>> V
>> but the problem is the same.
>
> I think I found a bug in the cgroup freezer which your patch fixes.
> However I don't think it's a race.
>
> /* Task is frozen or will freeze immediately when next it gets woken */
> static bool is_task_frozen_enough(struct task_struct *task)
> {
> return frozen(task) ||
> (task_is_stopped_or_traced(task) && freezing(task));
> }
>
> So it will only refuse to attach freezing tasks which have been stopped
> or traced! Yet for attach we need to refuse to move _any_ freezing tasks.
>
> Though stopped/traced _is_ relevant to the cgroup freezer state itself.
> If we uses frozen(task) || freezing(task) to determine whether a cgroup
> is frozen then it would be possible for the task to still be active
> when the cgroup is finally reported FROZEN. However that's not possible
> when the task is stopped/traced *and* freezing since even if woken it
> won't exit the kernel before entering the refrigerator.
>
>>> But, without having carefully investigated the details, this could just as easily happen
>>> with your patch:
>>>
>>> Time "bug" goes through these states cgroup code checks for these states
>>> -----------------------------------------------------------------------------------
>>> | is_freezing? Nope.
>>> | is_frozen? Nope.
>>> | freezing
>>> | <move>
>>> | frozen
>>> V
>>>
>> This can't happen as far as I know because there is cgroup_lock around the code in freezer_write()
>> and freezer_can_attach().
>> The task can't enter 'freezing' state when can_attach is executed.
>
> You're right, cgroup_mutex should protect against that. However presumably
> that same logic applies to the first case. So again I don't think the bug
> is a race.
>
> <snipped the remaining cases which should be the same>
>
> At this point I think the bug description in your patch needs to change
> but the patch itself is perfect. Assuming you agree with my assessment
> of the bug I think the process is: you'll rewrite the description, add -stable
> maintaners to your current Cc's (since this bug is simple, exists in previous
> versions, and is somewhat nasty), add:
>
> Reviewed-by: Matt Helsley <matthltc(a)us.ibm.com>
> Tested-by: Matt Helsley <matthltc(a)us.ibm.com>
>
> and send it to Andrew Morton. Hopefully then (if not before) Paul and Li
> will ack it.
>
> Thanks!
>
> Cheers,
> -Matt Helsley

I agree with your assessment, Matt. The wrong the definition of being 'frozen enough'
allows a task to sneak out of its freezing cgroup.
The problem is that I found another, closely related bug. Right now, I have
a new queue of 3 patches to fix both bugs in a more general setting. They are not well tested
yet but I'm fairly confident that they do the right thing. I'm going to test them tomorrow.
Do you still want me to push the first patch? I propose to let me work a bit on
the new patches and prepare the code for the incoming fix to the related bug.
What is your opinion?

Tomasz
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Matt Helsley on 12 Aug 2010 17:20
On Thu, Aug 12, 2010 at 02:53:25AM +0200, Tomasz Buchert wrote:
> Matt Helsley a �crit :
> > On Wed, Aug 11, 2010 at 09:35:43AM +0200, Tomasz Buchert wrote:
> >> Matt Helsley a �crit :
> >>> On Wed, Aug 11, 2010 at 12:18:44AM +0200, Tomasz Buchert wrote:
> >>>> Matt Helsley a �crit :
> >>>>> On Tue, Aug 10, 2010 at 09:53:21PM +0200, Tomasz Buchert wrote:
> >>>>>> Writing 'FROZEN' to freezer.state file does not
> >>>>>> forbid the task to be moved away from its cgroup
> >>>>>> (for a very short time). Nevertheless the moved task
> >>>>>> can become frozen OUTSIDE its cgroup which puts
> >>>>>> discussed task in a permanent 'D' state.
> >>>>>>
> >>>>>> This patch forbids migration of either FROZEN
> >>>>>> or FREEZING tasks.
> >>>>>>
> >>>>>> This behavior was observed and easily reproduced on
> >>>>>> a single core laptop. Program and instructions how
> >>>>>> to reproduce the bug can be fetched from:
> >>>>>> http://pentium.hopto.org/~thinred/repos/linux-misc/freezer_bug.c
> >>>>> Thanks for the report and the test code.
> >>>>>
> >>>>> I'm will try to reproduce this race in the next few hours and analyze
> >>>>> it since I'm not sure the patch really fixes the race -- it may only
> >>>>> make the race trigger less frequently.
> >>>>>
> >>>>> At the very least the patch won't break the current code since it's
> >>>>> essentially a more-strict version of is_task_frozen_enough() -- it lets
> >>>>> fewer tasks attach/detach to/from frozen cgroups.
> >>>>>
> >>>>> Cheers,
> >>>>> -Matt Helsley
> >>>> Hi Matt!
> >>>> I am a novice if it comes to the kernel and I find the cgroup_freezer
> >>>> code especially complicated, so definetely this may be not enough to fix that.
> >>>> Notice also that if you uncomment the line 55 in my testcase this will also
> >>>> trigger the race! This, however, makes sense since process may not be in the cgroup anymore
> >>>> and consequently won't be thawed.
> >>> OK, I triggered it with that. Interesting.
> >>>
> >> Good!
> >>
> >>>> I think that this patch fixes these problems because it does the flag checking in a right order:
> >>>> first freezing() is used and then frozen() which assures (see frozen_process()) that
> >>>> the race will not happen. Right? :)
> >>> I see what you mean. It still seems like it wouldn't actually fix the race -- just make it
> >>> harder to trigger. I think you're saying this is what happens without the patch:
> >>>
> >>> Time "bug" goes through these states cgroup code checks for these states
> >>> -----------------------------------------------------------------------------------
> >>> | freezing
> >>> | is_frozen? Nope.
> >>> | frozen
> >>> | is_freezing? Nope.
> >>> | <move>
> >>> V
> >>>
> >> My first scenario was a bit different:
> >> Time "bug" goes through these states cgroup code checks for these states
> >> -----------------------------------------------------------------------------------
> >> | freezing
> >> | is_task_frozen_enough? Nope.
> >> | <move>
> >> | frozen
> >> V
> >> but the problem is the same.
> >
> > I think I found a bug in the cgroup freezer which your patch fixes.
> > However I don't think it's a race.
> >
> > /* Task is frozen or will freeze immediately when next it gets woken */
> > static bool is_task_frozen_enough(struct task_struct *task)
> > {
> > return frozen(task) ||
> > (task_is_stopped_or_traced(task) && freezing(task));
> > }
> >
> > So it will only refuse to attach freezing tasks which have been stopped
> > or traced! Yet for attach we need to refuse to move _any_ freezing tasks.
> >
> > Though stopped/traced _is_ relevant to the cgroup freezer state itself.
> > If we uses frozen(task) || freezing(task) to determine whether a cgroup
> > is frozen then it would be possible for the task to still be active
> > when the cgroup is finally reported FROZEN. However that's not possible
> > when the task is stopped/traced *and* freezing since even if woken it
> > won't exit the kernel before entering the refrigerator.
> >
> >>> But, without having carefully investigated the details, this could just as easily happen
> >>> with your patch:
> >>>
> >>> Time "bug" goes through these states cgroup code checks for these states
> >>> -----------------------------------------------------------------------------------
> >>> | is_freezing? Nope.
> >>> | is_frozen? Nope.
> >>> | freezing
> >>> | <move>
> >>> | frozen
> >>> V
> >>>
> >> This can't happen as far as I know because there is cgroup_lock around the code in freezer_write()
> >> and freezer_can_attach().
> >> The task can't enter 'freezing' state when can_attach is executed.
> >
> > You're right, cgroup_mutex should protect against that. However presumably
> > that same logic applies to the first case. So again I don't think the bug
> > is a race.
> >
> > <snipped the remaining cases which should be the same>
> >
> > At this point I think the bug description in your patch needs to change
> > but the patch itself is perfect. Assuming you agree with my assessment
> > of the bug I think the process is: you'll rewrite the description, add -stable
> > maintaners to your current Cc's (since this bug is simple, exists in previous
> > versions, and is somewhat nasty), add:
> >
> > Reviewed-by: Matt Helsley <matthltc(a)us.ibm.com>
> > Tested-by: Matt Helsley <matthltc(a)us.ibm.com>
> >
> > and send it to Andrew Morton. Hopefully then (if not before) Paul and Li
> > will ack it.
> >
> > Thanks!
> >
> > Cheers,
> > -Matt Helsley
>
> I agree with your assessment, Matt. The wrong the definition of being 'frozen enough'
> allows a task to sneak out of its freezing cgroup.
> The problem is that I found another, closely related bug. Right now, I have
> a new queue of 3 patches to fix both bugs in a more general setting. They are not well tested
> yet but I'm fairly confident that they do the right thing. I'm going to test them tomorrow.
> Do you still want me to push the first patch? I propose to let me work a bit on
> the new patches and prepare the code for the incoming fix to the related bug.
> What is your opinion?

OK, I'll have a look at the 3 new patches and your test(s) then we can discuss
what to do.

Cheers,
-Matt
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
First  |  Prev  |  Next  |  Last
Pages: 1 2 3
Prev: [PATCH 1/4] perf ui: Make SPACE work as PGDN in all browsers
Next: [PATCH 4/4] perf ui hist browser: Fixup key bindings