linux-kernel - Re: [PATCH] cgroup/cpuset: fix circular locking dependency

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Date:   Sat, 16 Dec 2017 00:36:33 +0530
From:   Prateek Sood <prsood@...eaurora.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, avagin@...il.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, sramana@...eaurora.org
Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency

On 12/15/2017 06:52 PM, Tejun Heo wrote:
> Hello, Prateek.
> 
> On Fri, Dec 15, 2017 at 02:24:55PM +0530, Prateek Sood wrote:
>> Following are two ways to improve cgroup_transfer_tasks(). In
>> both cases task in PF_EXITING state would be left in source
>> cgroup. It would be removed from cgroup_exit() in exit path.
>>
>> diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
>> index 024085d..e2bdcdb 100644
>> --- a/kernel/cgroup/cgroup-v1.c
>> +++ b/kernel/cgroup/cgroup-v1.c
>> @@ -123,7 +123,10 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
>>          */
>>         do {
>>                 css_task_iter_start(&from->self, 0, &it);
>> -               task = css_task_iter_next(&it);
>> +               do {
>> +                       task = css_task_iter_next(&it);
>> +               } while (task && (task & PF_EXITING))
>> +
> 
> Yeah, this looks good to me.  We can't just make a single pass as in
> the other one because we can race aginst fork.  And PF_EXITING being
> left behind is what was happening previously too anyway.  They can't
> be moved.
> 
> Thanks.
> 

Thanks TJ for reviewing.

I will send a formal patch with the above approved approach.


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