From 4369ef3c3e9d3bd9b879580678778f558d481e90 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 8 Jan 2006 01:01:35 -0800 Subject: [PATCH] [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 Cc: Philippe Elie Cc: John Levon Signed-off-by: "Paul E. McKenney" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/oprofile/buffer_sync.c | 30 +++++++++++++++--------------- 1 file changed, 15 insertions(+), 15 deletions(-) diff --git a/drivers/oprofile/buffer_sync.c b/drivers/oprofile/buffer_sync.c index 531b07313141..b2e8e49c8659 100644 --- a/drivers/oprofile/buffer_sync.c +++ b/drivers/oprofile/buffer_sync.c @@ -43,13 +43,16 @@ static void process_task_mortuary(void); * 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 increment_tail(struct oprofile_cpu_buffer * b) */ 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_del(&task->tasks); - free_task(task); - } + list_splice_init(&dead_tasks, &local_dead_tasks); + list_splice_init(&dying_tasks, &dead_tasks); - list_for_each_safe(pos, pos2, &dying_tasks) { - task = list_entry(pos, struct task_struct, tasks); + spin_unlock_irqrestore(&task_mortuary, flags); + + list_for_each_entry_safe(task, ttask, &local_dead_tasks, tasks) { list_del(&task->tasks); - list_add_tail(&task->tasks, &dead_tasks); + free_task(task); } - - spin_unlock(&task_mortuary); } -- 2.20.1