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From: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>,
	Shuah Khan <shuah@kernel.org>, Kees Cook <kees@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Jan Kara <jack@suse.cz>, Aleksa Sarai <cyphar@cyphar.com>,
	Andrei Vagin <avagin@google.com>, Kirill Tkhai <tkhai@ya.ru>,
	Alexander Mikhalitsyn <alexander@mihalicyn.com>,
	Adrian Reber <areber@redhat.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-kselftest@vger.kernel.org,
	Mateusz Guzik <mjguzik@gmail.com>
Subject: Re: [PATCH v2 1/2] pid_namespace: allow opening pid_for_children before init was created
Date: Tue, 24 Feb 2026 17:35:48 +0100	[thread overview]
Message-ID: <f4def6e7-fe45-4e76-9917-aff0deba51d9@virtuozzo.com> (raw)
In-Reply-To: <aZ3L3cRL8AEmfQpP@redhat.com>



On 2/24/26 17:03, Oleg Nesterov wrote:
> On 02/24, Pavel Tikhomirov wrote:
>>
>> On 2/24/26 13:09, Oleg Nesterov wrote:
>>> On 02/23, Pavel Tikhomirov wrote:
>>>>
>>>> To avoid possible problems related to cpu/compiler optimizations around
>>>> ->child_reaper, let's use WRITE_ONCE (additional to task_list lock)
>>>> everywhere we write it and use READ_ONCE everywhere we read it without
>>>> explicit lock.
>>>
>>> Yes, this is what I meant... but I can never recall if READ_ONCE() alone
>>> is enough to make KCSAN happy...
>>
>> AFAICS this should be fine for memory safety of accesses to ->child_reaper.
>> I would love if someone more experienced in this area would confirm.
> 
> __READ_ONCE() uses volatile cast, DEFINE_TSAN_VOLATILE_READ_WRITE()
> will pass KCSAN_ACCESS_ATOMIC to check_access(), so it seems that
> READ_ONCE() should be enough...
> 
> But I am not sure, I don't really understand this code.
> 
>>> I won't insist, but I think it would be better to do this in a separate
>>> change for documenation purposes and for discussion.
>>
>> Ok, will do. It will be a bit ugly as I will first add READ_ONCE to the
>> pidns_for_children_get() and then remove the hunk with it in the next patch.
> 
> Agreed, this is ugly. I almost regret I mentioned _ONCE() in the previous
> discussions, I only tried to "nack" another read_lock(tasklist).

Heh, I missed that in pidns_for_children_get() we have read under tasklist
lock, so I don't need to add _ONCE there so it should not be that ugly =).

I will test and send v3 with prep patches soon.

> 
> So lets avoid a separate change and WRITE_ONCE()'s in copy_process/find_child_reaper,
> we can add them later if KCSAN complains, they are not needed for correctness.
> 
> But up to you, I am fine either way.

I also noticed that 
https://docs.kernel.org/dev-tools/kcsan.html#c.ASSERT_EXCLUSIVE_WRITER
asks to add ASSERT_EXCLUSIVE_WRITER, so I will try to accommodate that.

> 
>>> Perhaps something like the preparational patch below makes sense ? Not
>>> sure this is actually better...
>>
>> This looks more universal at least, as instead of two checks we have one in one
>> place. My only concern of putting the check where I put it was to avoid extra
>> idr_alloc_cyclic() + idr_remove(), if we already know we don't need it. But it's
>> only in last pid_namespace we can have ->child_reaper unset so we do alloc/remove
>> for all other namespaces anyway in error case, should not be a big deal.
> 
> Yes...
> 
>> I will add the preparation patches: for below patch and related to _ONCE.
> 
> Again, up to you. But either way it would be nice to have a comment or at
> least a note in the changelog to explain that this is also needed to avoid
> the race between alloc_pid() + fail and another alloc_pid(). This is subtle.

Yes I will try to emphasize this.

> 
> Oleg.
> 

Thank you!

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.



  reply	other threads:[~2026-02-24 16:36 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-02-23 20:01 [PATCH v2 0/2] pid_namespace: make init creation more flexible Pavel Tikhomirov
2026-02-23 20:01 ` [PATCH v2 1/2] pid_namespace: allow opening pid_for_children before init was created Pavel Tikhomirov
2026-02-24  7:02   ` Andrei Vagin
2026-02-24 10:37     ` Pavel Tikhomirov
2026-02-24 15:38       ` Andrei Vagin
2026-02-24 16:09         ` Oleg Nesterov
2026-02-24 12:09   ` Oleg Nesterov
2026-02-24 13:23     ` Pavel Tikhomirov
2026-02-24 16:03       ` Oleg Nesterov
2026-02-24 16:35         ` Pavel Tikhomirov [this message]
2026-02-23 20:01 ` [PATCH v2 2/2] selftests: Add tests for creating pidns init via setns Pavel Tikhomirov

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