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Date: Wed, 29 Jul 2015 19:30:54 -0400
From: Rich Felker <>
Subject: Re: New optimized normal-type mutex?

On Thu, Jul 30, 2015 at 12:11:15AM +0200, Jens Gustedt wrote:
> Hello,
> Am Mittwoch, den 29.07.2015, 14:09 +0200 schrieb Joakim Sindholt:
> > So he went on and suggested that a cas-less lock was possible with
> > a_fetch_add however I can't make it work and I don't think he can
> > either. His idea however is sound: the one who flips the sign bit takes
> > the lock. Based on that I've cobbled together a different lock that will
> > probably perform worse than this approach but none-the-less be correct
> > as far as I can tell.
> > 
> > The difference is that we consider the lock owner a waiter as well, thus
> > requiring a cas loop in the unlock function to remove itself, so to
> > speak, from the waiter count. a_fetch_and also turns into a cas loop so
> > I consider this fairly minor.
> > This makes the wait loop a little simpler while still maintaining a
> > waiter count and still only using one int.
> Nice ideas!
> After the recent discussion about the problems on x86_64 I was trying
> to come up with a simple lock for the atomics, and I came thinking
> along the same lines.

Unfortunately, discussion on IRC has revealed a potentially
show-stopping issue for merging the waiter count into the futex word:
arrival of new waiters causes EAGAIN from futex_wait. I don't know any
good way around this, but it's probably the reason designs like this
have not been popular before.


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