Re: [PATCH] cleanup and remove some extra spinlocks from rtmutex

[Oleg Nesterov <oleg@tv-sign.ru>]

Another question: why should we take ->pi_lock to modify rt_mutex's
->wait_list? It looks confusing and unneeded to me, because we already
hold ->wait_lock. For example, wakeup_next_waiter() takes current's
->pi_lock before plist_del(), which seems to be completely offtopic,
since current->pi_blocked_on has nothing to do with that rt_mutex.

Note also that ->pi_blocked_on is always modified while also holding
->pi_blocked_on->lock->wait_lock, and things like rt_mutex_top_waiter()
need ->wait_lock too, so I don't think we need ->pi_lock for ->wait_list.

In other words, could you please explain to me whether the patch below
correct or not?

Thanks,

Oleg.

--- 2.6.18-rc3/kernel/rtmutex.c~2_rtm	2006-08-13 19:07:45.000000000 +0400
+++ 2.6.18-rc3/kernel/rtmutex.c	2006-08-13 22:09:45.000000000 +0400
@@ -236,6 +236,10 @@ static int rt_mutex_adjust_prio_chain(st
 		goto out_unlock_pi;
 	}
 
+	/* Release the task */
+	spin_unlock_irqrestore(&task->pi_lock, flags);
+	put_task_struct(task);
+
 	top_waiter = rt_mutex_top_waiter(lock);
 
 	/* Requeue the waiter */
@@ -243,10 +247,6 @@ static int rt_mutex_adjust_prio_chain(st
 	waiter->list_entry.prio = task->prio;
 	plist_add(&waiter->list_entry, &lock->wait_list);
 
-	/* Release the task */
-	spin_unlock_irqrestore(&task->pi_lock, flags);
-	put_task_struct(task);
-
 	/* Grab the next task */
 	task = rt_mutex_owner(lock);
 	get_task_struct(task);
@@ -416,15 +416,15 @@ static int task_blocks_on_rt_mutex(struc
 	plist_node_init(&waiter->list_entry, current->prio);
 	plist_node_init(&waiter->pi_list_entry, current->prio);
 
+	current->pi_blocked_on = waiter;
+
+	spin_unlock_irqrestore(&current->pi_lock, flags);
+
 	/* Get the top priority waiter on the lock */
 	if (rt_mutex_has_waiters(lock))
 		top_waiter = rt_mutex_top_waiter(lock);
 	plist_add(&waiter->list_entry, &lock->wait_list);
 
-	current->pi_blocked_on = waiter;
-
-	spin_unlock_irqrestore(&current->pi_lock, flags);
-
 	if (waiter == rt_mutex_top_waiter(lock)) {
 		spin_lock_irqsave(&owner->pi_lock, flags);
 		plist_del(&top_waiter->pi_list_entry, &owner->pi_waiters);
@@ -472,11 +472,10 @@ static void wakeup_next_waiter(struct rt
 	struct task_struct *pendowner;
 	unsigned long flags;
 
-	spin_lock_irqsave(&current->pi_lock, flags);
-
 	waiter = rt_mutex_top_waiter(lock);
 	plist_del(&waiter->list_entry, &lock->wait_list);
 
+	spin_lock_irqsave(&current->pi_lock, flags);
 	/*
 	 * Remove it from current->pi_waiters. We do not adjust a
 	 * possible priority boost right now. We execute wakeup in the
@@ -530,8 +529,9 @@ static void remove_waiter(struct rt_mute
 	unsigned long flags;
 	int chain_walk = 0;
 
-	spin_lock_irqsave(&current->pi_lock, flags);
 	plist_del(&waiter->list_entry, &lock->wait_list);
+
+	spin_lock_irqsave(&current->pi_lock, flags);
 	waiter->task = NULL;
 	current->pi_blocked_on = NULL;
 	spin_unlock_irqrestore(&current->pi_lock, flags);