In computing, a futex (short for "fast userspace mutex") is a Linux kernel system call that programmers can use to implement basic locking, or as a building block for higher-level locking abstractions such as semaphores and POSIX mutexes or condition variables.
Contents
A futex consists of a kernelspace wait queue that is attached to an aligned integer in userspace. Multiple processes or threads operate on the integer entirely in userspace (using atomic operations to avoid interfering with one another), and only resort to relatively expensive system calls to request operations on the wait queue (for example to wake up waiting processes, or to put the current process on the wait queue). A properly programmed futex-based lock will not use system calls except when the lock is contended; since most operations do not require arbitration between processes, this will not happen in most cases.
History
On Linux, Hubertus Franke (IBM Thomas J. Watson Research Center), Matthew Kirkwood, Ingo Molnár (Red Hat) and Rusty Russell (IBM Linux Technology Center) originated the futex mechanism. Futexes appeared for the first time in version 2.5.7 of the Linux kernel development series; the semantics stabilized as of version 2.5.40, and futexes have been part of the Linux kernel mainline since the December 2003 release of 2.6.x stable kernel series.
In 2002 discussions took place on a proposal to make futexes accessible via the file system by creating a special node in /dev or /proc. However, Linus Torvalds strongly opposed this idea and rejected any related patches.
In May 2014 the CVE system announced a vulnerability discovered in the Linux kernel's futex subsystem that allowed denial-of-service attacks or local privilege escalation.
A technique very similar to the futex mechanism, called benaphore, was available at least since 1996 in the BeOS operating system, which Be Inc. originally wrote for BeBox hardware.
Operations
Futexes have two basic operations, WAIT and WAKE. A third operation called REQUEUE is available and functions as a more generic WAKE operation that can move threads between waiting queues.
WAIT(addr, val)addr is val, puts the current thread to sleep.
WAKE(addr, num)num number of threads waiting on the address addr.
CMP_REQUEUE(old_addr, new_addr, num_wake, num_move, val)old_addr is val, wakes num_wake threads waiting on the address old_addr, and enqueues num_move threads waiting on the address old_addr to now wait on the address new_addr.