Commit e4e1e808 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Paul E. McKenney
Browse files

rcu-tasks: Improve comments explaining tasks_rcu_exit_srcu purpose 



Make sure we don't need to look again into the depths of git blame in
order not to miss a subtle part about how rcu-tasks is dealing with
exiting tasks.

Suggested-by: default avatarBoqun Feng <boqun.feng@gmail.com>
Suggested-by: default avatarNeeraj Upadhyay <quic_neeraju@quicinc.com>
Suggested-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 9420fb93

kernel/rcu/tasks.h

+29 −8
Original line number Diff line number Diff line
@@ -827,11 +827,21 @@ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop) 
static void rcu_tasks_postscan(struct list_head *hop)

{

	/*

	 * Wait for tasks that are in the process of exiting.  This

	 * does only part of the job, ensuring that all tasks that were

	 * previously exiting reach the point where they have disabled

	 * preemption, allowing the later synchronize_rcu() to finish

	 * the job.

	 * Exiting tasks may escape the tasklist scan. Those are vulnerable

	 * until their final schedule() with TASK_DEAD state. To cope with

	 * this, divide the fragile exit path part in two intersecting

	 * read side critical sections:

	 *

	 * 1) An _SRCU_ read side starting before calling exit_notify(),

	 *    which may remove the task from the tasklist, and ending after

	 *    the final preempt_disable() call in do_exit().

	 *

	 * 2) An _RCU_ read side starting with the final preempt_disable()

	 *    call in do_exit() and ending with the final call to schedule()

	 *    with TASK_DEAD state.

	 *

	 * This handles the part 1). And postgp will handle part 2) with a

	 * call to synchronize_rcu().

	 */

	synchronize_srcu(&tasks_rcu_exit_srcu);

}
@@ -898,7 +908,10 @@ static void rcu_tasks_postgp(struct rcu_tasks *rtp) 
	 *

	 * In addition, this synchronize_rcu() waits for exiting tasks

	 * to complete their final preempt_disable() region of execution,

	 * cleaning up after the synchronize_srcu() above.

	 * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu),

	 * enforcing the whole region before tasklist removal until

	 * the final schedule() with TASK_DEAD state to be an RCU TASKS

	 * read side critical section.

	 */

	synchronize_rcu();

}
@@ -988,7 +1001,11 @@ void show_rcu_tasks_classic_gp_kthread(void) 
EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);

#endif // !defined(CONFIG_TINY_RCU)



/* Do the srcu_read_lock() for the above synchronize_srcu().  */

/*

 * Contribute to protect against tasklist scan blind spot while the

 * task is exiting and may be removed from the tasklist. See

 * corresponding synchronize_srcu() for further details.

 */

void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)

{

	preempt_disable();
@@ -996,7 +1013,11 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu) 
	preempt_enable();

}



/* Do the srcu_read_unlock() for the above synchronize_srcu().  */

/*

 * Contribute to protect against tasklist scan blind spot while the

 * task is exiting and may be removed from the tasklist. See

 * corresponding synchronize_srcu() for further details.

 */

void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)

{

	struct task_struct *t = current;