linux-kernel - [PATCH 1/2] fix the wrong comment on task_lock() nesting with tasklist

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Message-ID: <20250914110908.GA18769@redhat.com>
Date: Sun, 14 Sep 2025 13:09:08 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christian Brauner <brauner@...nel.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Mateusz Guzik <mjguzik@...il.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] fix the wrong comment on task_lock() nesting with
 tasklist_lock

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.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 include/linux/sched/task.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index ea41795a352b..8ff98b18b24b 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -210,9 +210,8 @@ static inline struct vm_struct *task_stack_vm_area(const struct task_struct *t)
  * 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)
 {
-- 
2.25.1.362.g51ebf55