Re: [PATCH 3/3] futex: fix miss ordered wakeups

On Thu, 22 May 2008, Daniel Walker wrote:

When the plist was added to futexes it added overhead to sort based
on priority for the futex waiters. If there is a miss order the value of
this, from my perspective, is lost. Since we don't re-order tasks
when their priority is changed after they sleep then we get a miss ordered
scenerio, and tasks aren't woken in priority order.

This is a solution looking for a problem.

Normal futexes have no ordering guarantees at all. There is no
mechanism to prevent lock stealing from lower priority tasks. So why
should we care about the once a year case, where a sleepers priority
is modified ?

If you need ordering guarantees then use PI futexes.

This patch corrects this issue, so the tasks are always woken in priority
order.

The patch corrects a non issue and introduces lock order issues:

+void futex_adjust_waiters(struct task_struct *p)
+{
+ spin_lock(&p->pi_lock);
+ spin_lock(&hb->lock);
...
+ spin_unlock(&hb->lock);
+ }
+ spin_unlock(&p->pi_lock);
+}

vs.

@@ -1155,6 +1191,8 @@ static int futex_wait(u32 __user *uaddr,
{
....
hb = queue_lock(&q);

+ spin_lock(&current->pi_lock);
+ current->blocked_on = &blocked_on;
+ spin_unlock(&current->pi_lock);

There are more issues vs. pi futexes as well. The simple case of
futex_wait() vs. futex_adjust_waiters will just upset lockdep, but
there are real dealocks vs. unqueue_me_pi waiting.

Thanks,
tglx

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