[PATCH 1/2] fix the wrong comment on task_lock() nesting with tasklist_lock

Oleg Nesterov
2025-09-14T11:10:43Z

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@redhat.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