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Commit 4369ef3c authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Linus Torvalds
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[PATCH] Make RCU task_struct safe for oprofile 



Applying RCU to the task structure broke oprofile, because
free_task_notify() can now be called from softirq.  This means that the
task_mortuary lock must be acquired with irq disabled in order to avoid
intermittent self-deadlock.  Since irq is now disabled, the critical
section within process_task_mortuary() has been restructured to be O(1) in
order to maximize scalability and minimize realtime latency degradation.

Kudos to Wu Fengguang for finding this problem!

CC: Wu Fengguang <wfg@mail.ustc.edu.cn>
Cc: Philippe Elie <phil.el@wanadoo.fr>
Cc: John Levon <levon@movementarian.org>
Signed-off-by: default avatar"Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent eafbaa94

drivers/oprofile/buffer_sync.c

+15 −15
Original line number Original line Diff line number Diff line
@@ -43,13 +43,16 @@ static void process_task_mortuary(void); 
 * list for processing. Only after two full buffer syncs

 * list for processing. Only after two full buffer syncs

 * does the task eventually get freed, because by then

 * does the task eventually get freed, because by then

 * we are sure we will not reference it again.

 * we are sure we will not reference it again.

 * Can be invoked from softirq via RCU callback due to

 * call_rcu() of the task struct, hence the _irqsave.

 */

 */

static int task_free_notify(struct notifier_block * self, unsigned long val, void * data)

static int task_free_notify(struct notifier_block * self, unsigned long val, void * data)

{

{

	unsigned long flags;

	struct task_struct * task = data;

	struct task_struct * task = data;

	spin_lock(&task_mortuary);

	spin_lock_irqsave(&task_mortuary, flags);

	list_add(&task->tasks, &dying_tasks);

	list_add(&task->tasks, &dying_tasks);

	spin_unlock(&task_mortuary);

	spin_unlock_irqrestore(&task_mortuary, flags);

	return NOTIFY_OK;

	return NOTIFY_OK;

}

}
@@ -431,25 +434,22 @@ static void increment_tail(struct oprofile_cpu_buffer * b) 
 */

 */

static void process_task_mortuary(void)

static void process_task_mortuary(void)

{

{

	struct list_head * pos;

	unsigned long flags;

	struct list_head * pos2;

	LIST_HEAD(local_dead_tasks);

	struct task_struct * task;

	struct task_struct * task;

	struct task_struct * ttask;





	spin_lock(&task_mortuary);

	spin_lock_irqsave(&task_mortuary, flags);





	list_for_each_safe(pos, pos2, &dead_tasks) {

	list_splice_init(&dead_tasks, &local_dead_tasks);

		task = list_entry(pos, struct task_struct, tasks);

	list_splice_init(&dying_tasks, &dead_tasks);

		list_del(&task->tasks);

		free_task(task);

	}





	list_for_each_safe(pos, pos2, &dying_tasks) {

	spin_unlock_irqrestore(&task_mortuary, flags);

		task = list_entry(pos, struct task_struct, tasks);



	list_for_each_entry_safe(task, ttask, &local_dead_tasks, tasks) {

		list_del(&task->tasks);

		list_del(&task->tasks);

		list_add_tail(&task->tasks, &dead_tasks);

		free_task(task);

	}

	}



	spin_unlock(&task_mortuary);

}

}