sched/task.h: 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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: oleg-nesterov

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)
 {