Re: [PATCH 1/1] perf/core: fix dangling cgroup pointer in cpuctx

["David Wang" <00107082@163.com>]

At 2025-06-03 15:16:36, "Yeoreum Yun" <yeoreum.yun@arm.com> wrote:
>Hi David,
>
>> >
>> > >
>> > > Also, your patch couldn't solve a problem describe in
>> > > commit a3c3c6667("perf/core: Fix child_total_time_enabled accounting bug at task exit")
>> > > for INCATIVE event's total_enable_time.
>> >
>> > I do not think so.
>> > Correct me if I am making silly  mistakes,
>> > The patch, https://lore.kernel.org/lkml/20250603032651.3988-1-00107082@163.com/
>> > calls perf_event_set_state() based on DETACH_EXIT flag, which cover the INACTIVE state, right?
>> > If DETACH_EXIT is not used for this purpose? Then why should it exist at the first place?
>> > I think I does not revert the purpose of commit a3c3c6667.....But I could be wrong
>> > Would you show a call path where DETACH_EXIT is not set, but the changes in commit a3c3c6667 is still needed?
>> >
>> > Sorry for my bad explaination without detail.
>> > Think about cpu specific event and closed by task.
>> > If there is specific child cpu event specified in cpu 0.
>> > 1. cpu 0 -> active
>> > 2. scheulded to cpu1 -> inactive
>> > 3. close the cpu event from parent -> inactive close
>> >
>> > Can be failed to count total_enable_time.
>>
>> Is this explaining the purpose of commit a3c3c6667 ?
>> I am not arguing with it. And I also not suggest reverting it. (it is just that reverting it can fix the kernel panic.)
>
>In commit a3c3c6667, I explain the specific case but not with above
>case. But the commit's purpose is "account total_enable_time" properly.
>
>> > And also, considering the your patch, for DETACH_EXIT case,
>> > If it changes the state before list_del_event() that wouldn't disable
>> > related to the cgroup. So it would make cpuctx->cgrp pointer could be dangled
>> > as patch describe...
>> No, I don't think so.
>> change state before list_del_event(), this is the same behavior before commit a3c3c6667, right?
>> And no such kernel panic happened  before commit a3c3c6667.

Oh! I was wrong, before commit a3c3c6667, "change state" happened *after* list_del_event()
>
>That's why list_del_event() handle the perf_cgroup_disable() before the
>commit a3c3c6667. However because of *my mistake*, I've forget to
>perf_cgroup_disable() properly before change the event state.
>Yes, your patch can make avoid the panic since as soon as exit,
>the event->cgrp switched.

 I cannot agree with the reasoning, 
The panic dose not happened when exit, it happened when reboot/shutdown.
(I close perf_event_open before reboot)





>
>However, as I said, the INACTIVE event could be failed to count
>total_enable_time.
>
>So, set event should be occured before list_del_event().
>And since it's event->state change on remove.
>It shouldn't have any side effect the state change isn't cause of your
>panic. But missed perf_cgroup_disable().

Any procedure to bring out the impact of this missed perf_cgroup_disable()?
My system seems all normal, where should I check it?

But to fix it,  isn't following change less aggressive?
        event_sched_out(event, ctx);
-       perf_event_set_state(event, min(event->state, state));
        if (flags & DETACH_GROUP)
                perf_group_detach(event);
        if (flags & DETACH_CHILD)
                perf_child_detach(event);
        list_del_event(event, ctx);
+       perf_event_set_state(event, min(event->state, state));
 

David


>
>Thanks.
>--
>Sincerely,
>Yeoreum Yun