Messages in this thread | | | Subject | Re: [PATCH 3/3] futex: fix miss ordered wakeups | From | Daniel Walker <> | Date | Sat, 24 May 2008 08:32:45 -0700 |
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On Sat, 2008-05-24 at 10:55 +0200, Thomas Gleixner wrote:
> 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 ?
Lock stealing? The usage of sched_setscheduler is fairly pervasive in userspace, if a task becomes SCHED_FIFO it did so via sched_setscheduler. So I don't think this is at all "once a year". Tasks shouldn't be forced to determine if a task is sleeping or not before it calls sched_setscheduler.
> If you need ordering guarantees then use PI futexes.
There are degree's of overhead with each step.. Someone may not need or want priority inheritance.
> > +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.
You mean the lock ordering would cause the deadlock vs. unqueue_me_pi , or are you talking about something else?
Daniel
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