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

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Date:	Sat, 24 May 2008 10:24:20 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] futex: fix miss ordered wakeups


On Sat, 2008-05-24 at 19:03 +0200, Thomas Gleixner wrote:
> 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.

Thomas if you want to be abusive, talk to someone else.

> 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

This is how I would imagine the pre-plist code would work.

> > > 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.

huh?

> > > 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 ?

I guess so .. 

Daniel

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