[davidtgoldblatt]

There's two I've tried to do this:

- On the wait side, do the CAS to set the "waiter present" bit. Down unlock, do a (relaxed) read of the lock word, and if "waiter present" isn't set, just do a release store to unlock (and go down some slow CAS-y wake path if a waiter is present). On the wait side, never do an un-timed futex wait; just do a series of timed waits, with increasing wait times (so that you still eventually fix things if you hit the unlucky race between the previous holder's check+store sequence). (You can also do some tricks with counters to let waiters do an unconditional sleep once they get their wait acknowledged).

- Split out the "waiter present" bit into its own byte, do a store-load sequence (with just a compiler reordering fence) to check for waiters, and have waiters either do a membarrier() syscall or wait "long enough" that they're sure they've gotten the same effect. (This gets tricky w.r.t. mutex lifetime though; you either need out of band lifetime knowledge or to use RCU or whatever and indirect through pointers).

Practically, neither was ever "better enough" for anything but microbenchmarks to be worth the complexity.

[gpderetta]

> Split out the "waiter present" bit into its own byte, do a store-load sequence (with just a compiler reordering fence) to check for waiters, and have waiters either do a membarrier() syscall or wait "long enough" that they're sure they've gotten the same effect. (This gets tricky w.r.t. mutex lifetime though; you either need out of band lifetime knowledge or to use RCU or whatever and indirect through pointers).

If you are doing this, given the cost of membarrier, you are optimizing for almost always uncontended locks. At this point you can make your lock default-owned by the first thread to lock it and have the owner lock and unlock be basically free until it is contended. This is basically the biased locking optimization that Java implements (or used to).

[davidtgoldblatt]

It kinda depends; you only do the membarrier when you're about to sleep anyways, and the non-expedited membarrier() call is just a synchronize_rcu(), so it's not that drastically more expensive than a futex wait.

You don't necessarily want a biased lock for all this kind of stuff, because "sparsely contended" doesn't necessarily imply thread-associated. E.g. one place I was looking at this for was locks for pages of virtual memory in a heap; no thread "owns" any given heap page, but it was very uncommon to get unlucky and have two threads touching adjacent pages at the exact same time. These kind of "sloppy mutexes" get half the fast-path speedup of biased locks but without the heavily asymmetric performance costs. (At least, that was the theory; like I said it didn't really pan out to be that useful in practice).