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

From: Esben Nielsen
Date: Mon Aug 14 2006 - 16:27:28 EST

On Sun, 13 Aug 2006, Oleg Nesterov wrote:

Another question: why should we take ->pi_lock to modify rt_mutex's
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.

Yes, that was the basic design:

lock->wait_list and related waiter->list_entry is protected by lock->wait_lock, while task->pi_waiters and related waiter->pi_list_entry.

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

Well, we are talking about small optimizations now, moving only a few instructions outside the lock. Except for one of them it is correct, but it is worth risking stability for now?



--- 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);

So you want the task to go away here and use it below?

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);

No! It is used in the line just above, so we better be sure it still exists!

/* Grab the next task */
task = rt_mutex_owner(lock);
@@ -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);

Ok, this change might work out.

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);

This might be ok, too...

* 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);

And ok.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at