+ fix-the-wrong-comment-on-task_lock-nesting-with-tasklist_lock.patch added to mm-nonmm-unstable branch

+ fix-the-wrong-comment-on-task_lock-nesting-with-tasklist_lock.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: sched/task.h: fix the wrong comment on task_lock() nesting with tasklist_lock
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     fix-the-wrong-comment-on-task_lock-nesting-with-tasklist_lock.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fix-the-wrong-comment-on-task_lock-nesting-with-tasklist_lock.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Oleg Nesterov <oleg@redhat.com>
Subject: sched/task.h: fix the wrong comment on task_lock() nesting with tasklist_lock
Date: Sun, 14 Sep 2025 13:09:08 +0200

The ancient comment above task_lock() states that it can be nested outside
of read_lock(&tasklist_lock), but this is no longer true:

  CPU_0			CPU_1			CPU_2

  task_lock()		read_lock(tasklist)
  						write_lock_irq(tasklist)
  read_lock(tasklist)	task_lock()

Unless CPU_0 calls read_lock() in IRQ context, queued_read_lock_slowpath()
won't get the lock immediately, it will spin waiting for the pending
writer on CPU_2, resulting in a deadlock.

Link: https://lkml.kernel.org/r/20250914110908.GA18769@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/sched/task.h |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/include/linux/sched/task.h~fix-the-wrong-comment-on-task_lock-nesting-with-tasklist_lock
+++ a/include/linux/sched/task.h
@@ -210,9 +210,8 @@ static inline struct vm_struct *task_sta
  * pins the final release of task.io_context.  Also protects ->cpuset and
  * ->cgroup.subsys[]. And ->vfork_done. And ->sysvshm.shm_clist.
  *
- * Nests both inside and outside of read_lock(&tasklist_lock).
- * It must not be nested with write_lock_irq(&tasklist_lock),
- * neither inside nor outside.
+ * Nests inside of read_lock(&tasklist_lock). It must not be nested with
+ * write_lock_irq(&tasklist_lock), neither inside nor outside.
  */
 static inline void task_lock(struct task_struct *p)
 {
_

Patches currently in -mm which might be from oleg@redhat.com are

fix-the-wrong-comment-on-task_lock-nesting-with-tasklist_lock.patch
fix-the-racy-usage-of-task_locktsk-group_leader-in-sys_prlimit64-paths.patch