Re: [PATCH next] sched,freezer: prevent tasks from escaping being frozen

[Chen Ridong]

On 2025/7/4 11:02, Chen Ridong wrote:
> 
> 
> On 2025/7/4 1:01, Michal Koutný wrote:
>> Hello Ridong.
>>
>> On Thu, Jul 03, 2025 at 01:34:27PM +0000, Chen Ridong <chenridong@xxxxxxxxxxxxxxx> wrote:
>>> 2. The cgroup freezer state changes to FROZEN (Can be triggered by reading
>>>    freezer.state).
>> /o\
>>
>>> 3. freezing() is called and returns false.
>>
>> I can see how this can happen because freezer_lock != freezer_mutex.
>>
>>> As a result, the task may escape being frozen when it should be.
>>>
>>> To fix this, move the setting of the FROZEN flag to occur just before
>>> schedule(). This ensures the flag is only set when we're certain the
>>> task must be switched out.
>>
>> Is it sufficient? (If the task is spuriously woken up, the next
>> iteration in that refrigerator loop would be subject to same race, no?)
>>
>> Thanks,
>> Michal
> 
> Hi, Michal:
> 
> Regarding your question: Did you mean that the task was frozen, received
> another signal to wake up, but should have remained frozen instead of
> entering the running state?
> 
> For this scenario, the solution I've found is that the task can only
> break out of the frozen state when its cgroup is thawed. The code
> modification would look like the following, and we'll need to add the
> cgroup_thawed(p) function:
> 

Sorry, the code should look like:

--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -71,19 +71,20 @@ bool __refrigerator(bool check_kthr_stop)
        for (;;) {
                bool freeze;

-               raw_spin_lock_irq(&current->pi_lock);
-               WRITE_ONCE(current->__state, TASK_FROZEN);
-               /* unstale saved_state so that __thaw_task() will wake
us up */
-               current->saved_state = TASK_RUNNING;
-               raw_spin_unlock_irq(&current->pi_lock);
-
                spin_lock_irq(&freezer_lock);
-               freeze = freezing(current) && !(check_kthr_stop &&
kthread_should_stop());
+               freeze = (freezing(current) || !cgroup_thawed(current))
+                        && !(check_kthr_stop && kthread_should_stop());
                spin_unlock_irq(&freezer_lock);

                if (!freeze)
                        break;

+               raw_spin_lock_irq(&current->pi_lock);
+               WRITE_ONCE(current->__state, TASK_FROZEN);
+               /* unstale saved_state so that __thaw_task() will wake
us up */
+               current->saved_state = TASK_RUNNING;
+               raw_spin_unlock_irq(&current->pi_lock);
+
                was_frozen = true;
                schedule();
        }

Best regards,
Ridong