linux-kernel - Re: [PATCH] Futex: Revert the non-functional REQUEUE

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Date:	Mon, 18 Jun 2007 12:58:26 +0200
From:	Pierre Peiffer <pierre.peiffer@...l.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Linus Torvalds <torvalds@...l.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...l.org>,
	Ulrich Drepper <drepper@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: [PATCH] Futex: Revert the non-functional REQUEUE_PI

Thomas Gleixner wrote :
> Patch d0aa7a70bf03b9de9e995ab272293be1f7937822 titled 
> 
> "futex_requeue_pi optimization"
> 
> introduced user space visible changes to the futex syscall.
> 
> The patch is non-functional and there is no way to fix it proper before
> the 2.6.22 release. 
> 
> The breakage report ( http://lkml.org/lkml/2007/5/12/17 ) went
> unanswered,

Sorry, but I passed lot of time on this last year without any answers or 
comments from the community when I have sent my patches.
Now I'm working on something else and I can't spent more time on this for now... 
Futexes are so complex that it is always difficult to investigate a problem in 
only one or two hours...

> and unfortunately it turned out that the concept is not
> feasible at all. 

Without robust futex, the concept works well, and the performance gain has been 
proven.

It violates the rtmutex semantics badly by introducing
> a virtual owner, which hacks around the coupling of the user-space
> pi_futex and the kernel internal rt_mutex representation.

What you call a hack, is for me a design point. And if this is a hack, then I 
think that we can say that futexes are based on a series of hacks...
But, okay, this is not very constructive, so...

Ulrich Drepper wrote :
 >
 > Indeed.  A lot more discussion is needed to handle this correctly.  No
 > committed code in glibc so far uses the function so removal is no problem.

Once again, it's a pity that people didn't spent time to comment on this when I 
was working on this.
Now, the work will probably just be lost...

-- 
Pierre
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