[PATCH v5 1/4] pid_namespace: avoid optimization of accesses to ->child_reaper

Pavel Tikhomirov
2026-03-13T11:12:08Z

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 <oleg@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
--
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