Date: Wed, 29 Jul 2015 19:30:54 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com 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. Rich
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