Messages in this thread | | | Date | Sat, 24 May 2008 19:03:43 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH 3/3] futex: fix miss ordered wakeups |
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On Sat, 24 May 2008, Daniel Walker wrote: > 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?
Do you have the faintest idea how the futex code works at all ? There is no guarantee that the task which is woken up first gets the futex.
A) A task on another CPU can get it independent of its priority B) In case of multiple waiters wakeup there is no guarantee either
> The usage of sched_setscheduler is fairly pervasive in > userspace, if a task becomes SCHED_FIFO it did so via > sched_setscheduler.
Sigh.
sched_setscheduler is usually done during the startup and not in the middle of some operation.
> 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.
A sane written program which uses RT priorities does none of this and I don't care about abstruse use cases at all.
> > 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.
Then there is no need to add this artifical "correctness" at all.
> > 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?
Do I write Chinese or what ?
Thanks, tglx
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