membarrier Private Expedited Orders Userspace Memory Across Threads
Linux membarrier() can move part of a synchronization cost from a frequently executed path to a less frequent coordination path. With MEMBARRIER_CMD_PRIVATE_EXPEDITED, one thread asks the kernel to establish a memory-ordering point across the running threads in the same process. The caller pays for the system call when coordination is needed instead of requiring every fast-path execution to carry an explicit hardware memory barrier. This is narrower than a general thread rendezvous. membarrier() does not run an application callback on sibling threads, does not wait for application-level acknowledgements, and does not turn ordinary data races into valid synchronization. Its contract concerns ordering of userspace memory accesses around the barrier.