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SubjectRe: [PATCH 3/3] cgroup/cpuset: Allow only one active attach operation per cpuset
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On 4/6/23 05:44, Christian Brauner wrote:
> On Fri, Mar 31, 2023 at 10:50:45AM -0400, Waiman Long wrote:
>> The current cpuset code uses the global cpuset_attach_old_cs variable
>> to store the old cpuset value between consecutive cpuset_can_attach()
>> and cpuset_attach() calls. Since a caller of cpuset_can_attach() may
>> not need to hold the global cgroup_threadgroup_rwsem, parallel cpuset
>> attach operations are possible.
>>
>> When there are concurrent cpuset attach operations in progress,
>> cpuset_attach() may fetch the wrong value from cpuset_attach_old_cs
>> causing incorrect result. To avoid this problem while still allowing
>> certain level of parallelism, drop cpuset_attach_old_cs and use a
>> per-cpuset attach_old_cs value. Also restrict to at most one active
>> attach operation per cpuset to avoid corrupting the value of the
>> per-cpuset attach_old_cs value.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>> kernel/cgroup/cpuset.c | 19 ++++++++++++++-----
>> 1 file changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 2367de611c42..3f925c261513 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -198,6 +198,8 @@ struct cpuset {
>>
>> /* Handle for cpuset.cpus.partition */
>> struct cgroup_file partition_file;
>> +
>> + struct cpuset *attach_old_cs;
>> };
>>
>> /*
>> @@ -2456,22 +2458,27 @@ static int fmeter_getrate(struct fmeter *fmp)
>> return val;
>> }
>>
>> -static struct cpuset *cpuset_attach_old_cs;
>> -
>> /* Called by cgroups to determine if a cpuset is usable; cpuset_rwsem held */
>> static int cpuset_can_attach(struct cgroup_taskset *tset)
>> {
>> struct cgroup_subsys_state *css;
>> - struct cpuset *cs;
>> + struct cpuset *cs, *oldcs;
>> struct task_struct *task;
>> int ret;
>>
>> /* used later by cpuset_attach() */
>> - cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
>> + oldcs = task_cs(cgroup_taskset_first(tset, &css));
>> cs = css_cs(css);
>>
>> percpu_down_write(&cpuset_rwsem);
>>
>> + /*
>> + * Only one cpuset attach operation is allowed for each cpuset.
>> + */
>> + ret = -EBUSY;
>> + if (cs->attach_in_progress)
>> + goto out_unlock;
> That'll mean CLONE_INTO_CGROUP becomes even more interestig because it
> isn't subject to this restriction in contrast to fork()+migrate, right?

This patch is not related to the CLONE_INTO_CGROUP. In fact, I have
dropped that in latter version.

Cheers,
Longman

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