Messages in this thread | | | Date | Sat, 24 May 2008 10:55:15 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH 3/3] futex: fix miss ordered wakeups |
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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(¤t->pi_lock); > + current->blocked_on = &blocked_on; > + spin_unlock(¤t->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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