[Devel] Re: [PATCH] cgroup_freezer: Freezing and task move race fix

[Tomasz Buchert]

Tomasz Buchert a écrit :
> Matt Helsley a écrit :
>> 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
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