[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: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/drivers/oprofile/buffer_sync.c b/drivers/oprofile/buffer_sync.c
index 531b073..b2e8e49 100644
--- a/drivers/oprofile/buffer_sync.c
+++ b/drivers/oprofile/buffer_sync.c
@@ -43,13 +43,16 @@
* list for processing. Only after two full buffer syncs
* does the task eventually get freed, because by then
* 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)
{
+ unsigned long flags;
struct task_struct * task = data;
- spin_lock(&task_mortuary);
+ spin_lock_irqsave(&task_mortuary, flags);
list_add(&task->tasks, &dying_tasks);
- spin_unlock(&task_mortuary);
+ spin_unlock_irqrestore(&task_mortuary, flags);
return NOTIFY_OK;
}
@@ -431,25 +434,22 @@
*/
static void process_task_mortuary(void)
{
- struct list_head * pos;
- struct list_head * pos2;
+ unsigned long flags;
+ LIST_HEAD(local_dead_tasks);
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) {
- task = list_entry(pos, struct task_struct, tasks);
+ list_splice_init(&dead_tasks, &local_dead_tasks);
+ list_splice_init(&dying_tasks, &dead_tasks);
+
+ spin_unlock_irqrestore(&task_mortuary, flags);
+
+ list_for_each_entry_safe(task, ttask, &local_dead_tasks, tasks) {
list_del(&task->tasks);
free_task(task);
}
-
- list_for_each_safe(pos, pos2, &dying_tasks) {
- task = list_entry(pos, struct task_struct, tasks);
- list_del(&task->tasks);
- list_add_tail(&task->tasks, &dead_tasks);
- }
-
- spin_unlock(&task_mortuary);
}