linux-kernel - Re: [PATCH 6/6] futex: fix miss ordered wakeups

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Message-Id: <1214412463.21035.57.camel@localhost.localdomain>
Date:	Wed, 25 Jun 2008 09:47:43 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 6/6] futex: fix miss ordered wakeups

On Wed, 2008-06-25 at 18:17 +0200, Peter Zijlstra wrote:

> You're not the maintainer, and you fail to respect their opinion - so
> what makes you think your patches are going anywhere but /dev/null?

It may not be going anyplace. I'm not submitting it to anyone but LKML,
at this point anyway.

> Also, the main point was about mixing user and kernel space state, you
> still do so by including the futex waiter in the same union. That's a
> fundamental fugly - no matter if you can make it work.

I don't think it's ugly at all, but I'm open to suggestion for alternate
methods of implementing it .. I don't need to unify the blocked_on
structures, but it does allow for some nice things like reducing the
size of the task struct, and potentially later doing PI across different
API's.

> > > > in fact the problem is
> > > > whether or not the changes are needed (not weather their broken).. I
> > > > gave justification in the last thread, and I'm not sure why it's unclear
> > > > to you..
> > > 
> > > You failed to convince, also justification goes in the changelog, not in
> > > random lkml threads.
> > 
> > It boils down to POSIX compliance which was discussed in the last
> > thread. POSIX requires the waiters to be sorts for 5-10 different API's
> > which ultimately use the futex (most of which aren't at all related to
> > PI).
> 
> I'm unconvinced, my reading of the spec doesn't say that at all. It says
> its up to how things get scheduled.

Any way you read it there is an ordering that isn't random or FIFO or
un-ordered, which is what we end up with now depending on the order the
syscalls are used.

We already go to the trouble of sorting the waiters, which I think
speaks for itself .. 

> Also, you have failed to say what real world use cases care about this
> behaviour. This was asked multiple times - you never answered any of
> those queries.

At this point it all seems academic .. We all know how the code works
now, and the issue I'm addressing .. There's no crash associated with
this ..

I don't have access to code where this causes a devastating problem, all
I have is people asking me about the lack of determinism in this
interface.

Daniel

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