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From: Oleg Nesterov <oleg@redhat.com>
To: Pavel Tikhomirov <ptikhomirov@virtuozzo.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 17:03:41 +0100	[thread overview]
Message-ID: <aZ3L3cRL8AEmfQpP@redhat.com> (raw)
In-Reply-To: <3f095a91-f052-4f38-a8e4-2e6dbc9a0c6a@virtuozzo.com>

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).

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.

> > 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.

Oleg.



  reply	other threads:[~2026-02-24 16:04 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 [this message]
2026-02-24 16:35         ` Pavel Tikhomirov
2026-02-23 20:01 ` [PATCH v2 2/2] selftests: Add tests for creating pidns init via setns Pavel Tikhomirov

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