linux-kernel - Re: [patch 1/1] schedule removal of FUTEX

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Message-Id: <20061031172312.79748be5.akpm@osdl.org>
Date:	Tue, 31 Oct 2006 17:23:12 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	linux-kernel@...r.kernel.org, drepper@...hat.com, mingo@...e.hu,
	tglx@...utronix.de
Subject: Re: [patch 1/1] schedule removal of FUTEX_FD

On Wed, 01 Nov 2006 12:19:05 +1100
Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Tue, 2006-10-31 at 15:09 -0800, akpm@...l.org wrote:
> > From: Andrew Morton <akpm@...l.org>
> > 
> > Apparently FUTEX_FD is unfixably racy and nothing uses it (or if it does, it
> > shouldn't).
> > 
> > Add a warning printk, give any remaining users six months to migrate off it.
> 
> This makes sense.  FUTEX_FD was for the NGPT project which did userspace
> threading, and hence couldn't block.  It was always kind of a hack
> (although unfixably racy isn't quite right, it depends on usage).
> 
> However, the existence of FUTEX_FD is what made Ingo complain that we
> couldn't simply pin the futex page in memory, because now a process
> could pin one page per fd.  Removing it would seem to indicate that we
> can return to a much simpler scheme of (1) pinning a page when someone
> does futex_wait, and (2) simply comparing futexes by physical address.
> 
> Now, I realize with some dismay that simplicity is no longer a futex
> feature, but it might be worth considering?

Sure.  Perhaps we could accelerate the removal schedule if we want to do
this.  Let's see how many 2.6.19 users squeak first.

> Cheers,
> Rusty.
> PS.  I used to have a patch for "ratelim_printk()" which hashed on the
> format string to reduce the chance that one message limit would clobber
> other messages.  I'll dig it out...

I think the caller-provided-state thing will work OK?
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