Cedric Le Goater wrote:
> Li Zefan wrote:
>> It is sufficient to check if @task is frozen, and no need to check if
>> the original freezer is frozen.
>
> hmm, a task being frozen does not mean that its freezer cgroup is
> frozen.
If a task has being frozen, then can_attach() returns -EBUSY at once.
If a task isn't frozen, then we have orig_freezer->state == THAWED.
So we need to check the state of the task only.
> So the extra check seems valid but looking at the comment :
>
> /*
> * The call to cgroup_lock() in the freezer.state write method prevents
> * a write to that file racing against an attach, and hence the
> * can_attach() result will remain valid until the attach completes.
> */
> static int freezer_can_attach(struct cgroup_subsys *ss,
>
> how do we know that the task_freezer(task), which is not protected by
> the cgroup_lock(), is not going to change its state to CGROUP_FROZEN
> it looks racy.
>
Since freezer_can_attach() is called by cgroup core with cgroup_lock held,
there is
no race with task attaching and state changing.
> C.
>
>> Signed-off-by: Li Zefan <[EMAIL PROTECTED]>
>> ---
>> kernel/cgroup_freezer.c | 16 +++++++---------
>> 1 files changed, 7 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
>> index 7f54d1c..6fadafe 100644
>> --- a/kernel/cgroup_freezer.c
>> +++ b/kernel/cgroup_freezer.c
>> @@ -162,9 +162,13 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
>> struct task_struct *task)
>> {
>> struct freezer *freezer;
>> - int retval;
>>
>> - /* Anything frozen can't move or be moved to/from */
>> + /*
>> + * Anything frozen can't move or be moved to/from.
>> + *
>> + * Since orig_freezer->state == FROZEN means that @task has been
>> + * frozen, so it's sufficient to check the latter condition.
>> + */
>>
>> if (is_task_frozen_enough(task))
>> return -EBUSY;
>> @@ -173,13 +177,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
>> if (freezer->state == CGROUP_FROZEN)
>> return -EBUSY;
>>
>> - retval = 0;
>> - task_lock(task);
>> - freezer = task_freezer(task);
>> - if (freezer->state == CGROUP_FROZEN)
>> - retval = -EBUSY;
>> - task_unlock(task);
>> - return retval;
>> + return 0;
>> }
>>
>> static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
>
>
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