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

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Message-ID: <alpine.LFD.1.10.0805241852490.3295@apollo.tec.linutronix.de>
Date:	Sat, 24 May 2008 19:03:43 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Walker <dwalker@...sta.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] futex: fix miss ordered wakeups

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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