> 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).
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).