Originální popis anglicky:
futex - Fast Userspace Locking
Návod, kniha: Linux Programmer's Manual
#include <linux/futex.h>
The Linux kernel provides futexes ('Fast Userspace muTexes') as a building block
for fast userspace locking and semaphores. Futexes are very basic and lend
themselves well for building higher level locking abstractions such as POSIX
mutexes.
This page does not set out to document all design decisions but restricts itself
to issues relevant for application and library development. Most programmers
will in fact not be using futexes directly but instead rely on system
libraries built on them, such as the NPTL pthreads implementation.
A futex is identified by a piece of memory which can be shared between different
processes. In these different processes, it need not have identical addresses.
In its bare form, a futex has semaphore semantics; it is a counter that can be
incremented and decremented atomically; processes can wait for the value to
become positive.
Futex operation is entirely userspace for the non-contended case. The kernel is
only involved to arbitrate the contended case. As any sane design will strive
for non-contension, futexes are also optimised for this situation.
In its bare form, a futex is an aligned integer which is only touched by atomic
assembler instructions. Processes can share this integer over mmap, via shared
segments or because they share memory space, in which case the application is
commonly called multithreaded.
Any futex operation starts in userspace, but it may necessary to communicate
with the kernel using the
futex(2) system call.
To 'up' a futex, execute the proper assembler instructions that will cause the
host CPU to atomically increment the integer. Afterwards, check if it has in
fact changed from 0 to 1, in which case there were no waiters and the
operation is done. This is the non-contended case which is fast and should be
common.
In the contended case, the atomic increment changed the counter from -1 (or some
other negative number). If this is detected, there are waiters. Userspace
should now set the counter to 1 and instruct the kernel to wake up any waiters
using the FUTEX_WAKE operation.
Waiting on a futex, to 'down' it, is the reverse operation. Atomically decrement
the counter and check if it changed to 0, in which case the operation is done
and the futex was uncontended. In all other circumstances, the process should
set the counter to -1 and request that the kernel wait for another process to
up the futex. This is done using the FUTEX_WAIT operation.
The futex system call can optionally be passed a timeout specifying how long the
kernel should wait for the futex to be upped. In this case, semantics are more
complex and the programmer is referred to
futex(2) for more details.
The same holds for asynchronous futex waiting.
To reiterate, bare futexes are not intended as an easy to use abstraction for
end-users. Implementors are expected to be assembly literate and to have read
the sources of the futex userspace library referenced below.
This man page illustrates the most common use of the
futex(2) primitives:
it is by no means the only one.
Futexes were designed and worked on by Hubertus Franke (IBM Thomas J. Watson
Research Center), Matthew Kirkwood, Ingo Molnar (Red Hat) and Rusty Russell
(IBM Linux Technology Center). This page written by bert hubert.
Initial futex support was merged in Linux 2.5.7 but with different semantics
from those described above. Current semantics are available from Linux 2.5.40
onwards.
futex(2), `Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux'
(proceedings of the Ottawa Linux Symposium 2002), futex example library,
futex-*.tar.bz2
<URL:ftp://ftp.kernel.org:/pub/linux/kernel/people/rusty/>.
