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Lelli , Vincent Guittot , Jan Kara , Oleg Nesterov , Aleksa Sarai , Andrei Vagin , Kirill Tkhai , Alexander Mikhalitsyn , Adrian Reber , Pavel Tikhomirov , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: [PATCH v5 1/4] pid_namespace: avoid optimization of accesses to ->child_reaper Date: Fri, 13 Mar 2026 12:09:39 +0100 Message-ID: <20260313111014.2068913-2-ptikhomirov@virtuozzo.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260313111014.2068913-1-ptikhomirov@virtuozzo.com> References: <20260313111014.2068913-1-ptikhomirov@virtuozzo.com> Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: FR4P281CA0190.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:ca::15) To PAWPR08MB9008.eurprd08.prod.outlook.com (2603:10a6:102:341::8) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PAWPR08MB9008:EE_|DB9PR08MB11361:EE_ X-MS-Office365-Filtering-Correlation-Id: 4424544e-d550-4caa-6070-08de80f1245b 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 DwApJjY7 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: To avoid potential 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 where we read it without explicit lock. Note: It also pairs with existing READ_ONCE with no lock in nsfs_fh_to_dentry(). Also let's add ASSERT_EXCLUSIVE_WRITER before write to identify to KCSAN that we don't expect any concurrent ->child_reaper modifications, and those must be detected. Suggested-by: Oleg Nesterov Acked-by: Oleg Nesterov Signed-off-by: Pavel Tikhomirov -- v3: Split from main commit. Add ASSERT_EXCLUSIVE_WRITER. v5: Add one more READ_ONCE for access without lock in free_pid(). --- kernel/exit.c | 3 ++- kernel/fork.c | 5 ++++- kernel/pid.c | 4 ++-- 3 files changed, 8 insertions(+), 4 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 1f32023d0dbe..25e9cb6de7e7 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -608,7 +608,8 @@ static struct task_struct *find_child_reaper(struct task_struct *father, reaper = find_alive_thread(father); if (reaper) { - pid_ns->child_reaper = reaper; + ASSERT_EXCLUSIVE_WRITER(pid_ns->child_reaper); + WRITE_ONCE(pid_ns->child_reaper, reaper); return reaper; } diff --git a/kernel/fork.c b/kernel/fork.c index 7d369a8a2ad0..19a75a041e6a 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2428,7 +2428,10 @@ __latent_entropy struct task_struct *copy_process( init_task_pid(p, PIDTYPE_SID, task_session(current)); if (is_child_reaper(pid)) { - ns_of_pid(pid)->child_reaper = p; + struct pid_namespace *ns = ns_of_pid(pid); + + ASSERT_EXCLUSIVE_WRITER(ns->child_reaper); + WRITE_ONCE(ns->child_reaper, p); p->signal->flags |= SIGNAL_UNKILLABLE; } p->signal->shared_pending.signal = delayed.signal; diff --git a/kernel/pid.c b/kernel/pid.c index 2f1dbcbc2349..bb2bb6c16498 100644 --- a/kernel/pid.c +++ b/kernel/pid.c @@ -128,7 +128,7 @@ void free_pid(struct pid *pid) * is the reaper wake up the reaper. The reaper * may be sleeping in zap_pid_ns_processes(). */ - wake_up_process(ns->child_reaper); + wake_up_process(READ_ONCE(ns->child_reaper)); break; case PIDNS_ADDING: /* Only possible if the 1st fork fails */ @@ -218,7 +218,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid, * Also fail if a PID != 1 is requested and * no PID 1 exists. */ - if (tid != 1 && !tmp->child_reaper) + if (tid != 1 && !READ_ONCE(tmp->child_reaper)) goto out_abort; retval = -EPERM; if (!checkpoint_restore_ns_capable(tmp->user_ns)) -- 2.53.0