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

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Message-ID: <5d17f1d7.666d.197348b78d1.Coremail.00107082@163.com>
Date: Tue, 3 Jun 2025 14:47:25 +0800 (CST)
From: "David Wang" <00107082@....com>
To: "Yeoreum Yun" <yeoreum.yun@....com>
Cc: peterz@...radead.org, mingo@...hat.com, mingo@...nel.org,
	acme@...nel.org, namhyung@...nel.org, leo.yan@....com,
	mark.rutland@....com, alexander.shishkin@...ux.intel.com,
	jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] perf/core: fix dangling cgroup pointer in cpuctx


At 2025-06-03 14:39:33, "Yeoreum Yun" <yeoreum.yun@....com> wrote:
>> > > attach_state doesn't related for event->state change.
>> > > if one event already cleared PERF_ATTACH_CONTEXT, that event is called
>> > > via list_del_event()
>> >
>> > Maybe this concern could be clarified, what about other subtle impacts.
>> > The change should be thorough reviewed, if you want to push it further.
>> >
>> > It takes me more than a month to figure out a procedure to reproduce the kernel panic bug,
>> > It is  just very hard to capture a bug happens in rare situation.
>> >
>> > And your patch has a global impact, it changes behavior unnecessarily.
>>
>> TBH, this patch just change of time of "event->state" while doing,
>> As my bad miss the disable cgorup perf.
>> I think there seems no other side effect for chaning state while in
>> removing event.
>> But, Let's wait for other people's review.
>>
>> > >
>> > > 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.
>>
>> Thanks.
>
>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.

>
>> --
>> Sincerely,
>> Yeoreum Yun
>
>--
>Sincerely,
>Yeoreum Yun