futex(3) man page, final draft for pre-release review
From: Michael Kerrisk (man-pages)
Date: Tue Dec 15 2015 - 08:44:03 EST
Hello all,
After much too long a time, the revised futex man page *will*
go out in the next man pages release (it has been merged
into master).
There are various places where the page could still be improved,
but it is much better (and more than 5 times longer) than the
existing page.
The rendered version of the page is shown below, so that people
can make any final comments/suggestions for improvements
before the release (but of course I'll also take any
improvements after release as well). The page source is
available from the Git repo
(http://git.kernel.org/cgit/docs/man-pages/man-pages.git).
As I mention above, there are various places where the page
could still be better, so the rendered text below is annotated
with some FIXMEs, in case anyone wants to address these before
release.
Thanks
Michael
NAME
futex - fast user-space locking
SYNOPSIS
#include <linux/futex.h>
#include <sys/time.h>
int futex(int *uaddr, int futex_op, int val,
const struct timespec *timeout, /* or: uint32_t val2 */
int *uaddr2, int val3);
Note: There is no glibc wrapper for this system call; see NOTES.
DESCRIPTION
The futex() system call provides a method for waiting until a
certain condition becomes true. It is typically used as a blockâ
ing construct in the context of shared-memory synchronization.
When using futexes, the majority of the synchronization operaâ
tions are performed in user space. The user-space program
employs the futex() system call only when it is likely that the
program has to block for a longer time until the condition
becomes true. The program uses another futex() operation to wake
anyone waiting for a particular condition.
A futex is a 32-bit valueâreferred to below as a futex wordâwhose
address is supplied to the futex() system call. (Futexes are 32
bits in size on all platforms, including 64-bit systems.) All
futex operations are governed by this value. In order to share a
futex between processes, the futex is placed in a region of
shared memory, created using (for example) mmap(2) or shmat(2).
(Thus, the futex word may have different virtual addresses in
different processes, but these addresses all refer to the same
location in physical memory.) In a multithreaded program, it is
sufficient to place the futex word in a global variable shared by
all threads.
When executing a futex operation that requests to block a thread,
the kernel will block only if the futex word has the value that
the calling thread supplied (as one of the arguments of the
futex() call) as the expected value of the futex word. The loadâ
ing of the futex word's value, the comparison of that value with
the expected value, and the actual blocking will happen atomiâ
FIXME: for next line, it would be good to have an explanation of
"totally ordered" somewhere around here.
cally and totally ordered with respect to concurrently executing
futex operations on the same futex word. Thus, the futex word is
used to connect the synchronization in user space with the impleâ
mentation of blocking by the kernel. Analogously to an atomic
compare-and-exchange operation that potentially changes shared
memory, blocking via a futex is an atomic compare-and-block operâ
ation.
One use of futexes is for implementing locks. The state of the
lock (i.e., acquired or not acquired) can be represented as an
atomically accessed flag in shared memory. In the uncontended
case, a thread can access or modify the lock state with atomic
instructions, for example atomically changing it from not
acquired to acquired using an atomic compare-and-exchange
instruction. (Such instructions are performed entirely in user
mode, and the kernel maintains no information about the lock
state.) On the other hand, a thread may be unable to acquire a
lock because it is already acquired by another thread. It then
may pass the lock's flag as a futex word and the value representâ
ing the acquired state as the expected value to a futex() wait
operation. This futex() call will block if and only if the lock
is still acquired. When releasing the lock, a thread has to
first reset the lock state to not acquired and then execute a
futex operation that wakes threads blocked on the lock flag used
as a futex word (this can be be further optimized to avoid unnecâ
essary wake-ups). See futex(7) for more detail on how to use
futexes.
Besides the basic wait and wake-up futex functionality, there are
further futex operations aimed at supporting more complex use
cases.
Note that no explicit initialization or destruction are necessary
to use futexes; the kernel maintains a futex (i.e., the kernel-
internal implementation artifact) only while operations such as
FUTEX_WAIT, described below, are being performed on a particular
futex word.
Arguments
The uaddr argument points to the futex word. On all platforms,
futexes are four-byte integers that must be aligned on a four-
byte boundary. The operation to perform on the futex is speciâ
fied in the futex_op argument; val is a value whose meaning and
purpose depends on futex_op.
The remaining arguments (timeout, uaddr2, and val3) are required
only for certain of the futex operations described below. Where
one of these arguments is not required, it is ignored.
For several blocking operations, the timeout argument is a
pointer to a timespec structure that specifies a timeout for the
operation. However, notwithstanding the prototype shown above,
for some operations, the least significant four bytes are used as
an integer whose meaning is determined by the operation. For
these operations, the kernel casts the timeout value first to
unsigned long, then to uint32_t, and in the remainder of this
page, this argument is referred to as val2 when interpreted in
this fashion.
Where it is required, the uaddr2 argument is a pointer to a secâ
ond futex word that is employed by the operation. The interpreâ
tation of the final integer argument, val3, depends on the operaâ
tion.
Futex operations
The futex_op argument consists of two parts: a command that specâ
ifies the operation to be performed, bit-wise ORed with zero or
or more options that modify the behaviour of the operation. The
options that may be included in futex_op are as follows:
FUTEX_PRIVATE_FLAG (since Linux 2.6.22)
This option bit can be employed with all futex operations.
It tells the kernel that the futex is process-private and
not shared with another process (i.e., it is being used
for synchronization only between threads of the same
process). This allows the kernel to make some additional
performance optimizations.
As a convenience, <linux/futex.h> defines a set of conâ
stants with the suffix _PRIVATE that are equivalents of
all of the operations listed below, but with the
FUTEX_PRIVATE_FLAG ORed into the constant value. Thus,
there are FUTEX_WAIT_PRIVATE, FUTEX_WAKE_PRIVATE, and so
on.
FUTEX_CLOCK_REALTIME (since Linux 2.6.28)
This option bit can be employed only with the
FUTEX_WAIT_BITSET and FUTEX_WAIT_REQUEUE_PI operations.
If this option is set, the kernel treats timeout as an
absolute time based on CLOCK_REALTIME.
If this option is not set, the kernel treats timeout as
relative time, measured against the CLOCK_MONOTONIC clock.
The operation specified in futex_op is one of the following:
FUTEX_WAIT (since Linux 2.6.0)
This operation tests that the value at the futex word
pointed to by the address uaddr still contains the
expected value val, and if so, then sleeps waiting for a
FUTEX_WAKE operation on the futex word. The load of the
value of the futex word is an atomic memory access (i.e.,
using atomic machine instructions of the respective archiâ
tecture). This load, the comparison with the expected
value, and starting to sleep are performed atomically and
totally ordered with respect to other futex operations on
the same futex word. If the thread starts to sleep, it is
considered a waiter on this futex word. If the futex
value does not match val, then the call fails immediately
with the error EAGAIN.
The purpose of the comparison with the expected value is
to prevent lost wake-ups. If another thread changed the
value of the futex word after the calling thread decided
to block based on the prior value, and if the other thread
executed a FUTEX_WAKE operation (or similar wake-up) after
the value change and before this FUTEX_WAIT operation,
then the latter will observe the value change and will not
start to sleep.
If the timeout argument is non-NULL, its contents specify
a relative timeout for the wait, measured according to the
CLOCK_MONOTONIC clock. (This interval will be rounded up
to the system clock granularity, and is guaranteed not to
expire early.) If timeout is NULL, the call blocks indefâ
initely.
The arguments uaddr2 and val3 are ignored.
FUTEX_WAKE (since Linux 2.6.0)
This operation wakes at most val of the waiters that are
waiting (e.g., inside FUTEX_WAIT) on the futex word at the
address uaddr. Most commonly, val is specified as either
1 (wake up a single waiter) or INT_MAX (wake up all waitâ
ers). No guarantee is provided about which waiters are
awoken (e.g., a waiter with a higher scheduling priority
is not guaranteed to be awoken in preference to a waiter
with a lower priority).
The arguments timeout, uaddr2, and val3 are ignored.
FUTEX_FD (from Linux 2.6.0 up to and including Linux 2.6.25)
This operation creates a file descriptor that is associâ
ated with the futex at uaddr. The caller must close the
returned file descriptor after use. When another process
or thread performs a FUTEX_WAKE on the futex word, the
file descriptor indicates as being readable with
select(2), poll(2), and epoll(7)
The file descriptor can be used to obtain asynchronous
notifications: if val is nonzero, then, when another
process or thread executes a FUTEX_WAKE, the caller will
receive the signal number that was passed in val.
The arguments timeout, uaddr2 and val3 are ignored.
Because it was inherently racy, FUTEX_FD has been removed
from Linux 2.6.26 onward.
FUTEX_REQUEUE (since Linux 2.6.0)
This operation performs the same task as FUTEX_CMP_REQUEUE
(see below), except that no check is made using the value
in val3. (The argument val3 is ignored.)
FUTEX_CMP_REQUEUE (since Linux 2.6.7)
This operation first checks whether the location uaddr
still contains the value val3. If not, the operation
fails with the error EAGAIN. Otherwise, the operation
wakes up a maximum of val waiters that are waiting on the
futex at uaddr. If there are more than val waiters, then
the remaining waiters are removed from the wait queue of
the source futex at uaddr and added to the wait queue of
the target futex at uaddr2. The val2 argument specifies
an upper limit on the number of waiters that are requeued
to the futex at uaddr2.
The load from uaddr is an atomic memory access (i.e.,
using atomic machine instructions of the respective archiâ
tecture). This load, the comparison with val3, and the
requeueing of any waiters are performed atomically and
totally ordered with respect to other operations on the
same futex word.
Typical values to specify for val are 0 or or 1. (Speciâ
fying INT_MAX is not useful, because it would make the
FUTEX_CMP_REQUEUE operation equivalent to FUTEX_WAKE.)
The limit value specified via val2 is typically either 1
or INT_MAX. (Specifying the argument as 0 is not useful,
because it would make the FUTEX_CMP_REQUEUE operation
equivalent to FUTEX_WAIT.)
The FUTEX_CMP_REQUEUE operation was added as a replacement
for the earlier FUTEX_REQUEUE. The difference is that the
check of the value at uaddr can be used to ensure that
requeueing happens only under certain conditions, which
allows race conditions to be avoided in certain use cases.
Both FUTEX_REQUEUE and FUTEX_CMP_REQUEUE can be used to
avoid "thundering herd" wake-ups that could occur when
using FUTEX_WAKE in cases where all of the waiters that
are woken need to acquire another futex. Consider the
following scenario, where multiple waiter threads are
waiting on B, a wait queue implemented using a futex:
lock(A)
while (!check_value(V)) {
unlock(A);
block_on(B);
lock(A);
};
unlock(A);
If a waker thread used FUTEX_WAKE, then all waiters waitâ
ing on B would be woken up, and they would would all try
to acquire lock A. However, waking all of the threads in
this manner would be pointless because all except one of
the threads would immediately block on lock A again. By
contrast, a requeue operation wakes just one waiter and
moves the other waiters to lock A, and when the woken
waiter unlocks A then the next waiter can proceed.
FUTEX_WAKE_OP (since Linux 2.6.14)
This operation was added to support some user-space use
cases where more than one futex must be handled at the
same time. The most notable example is the implementation
of pthread_cond_signal(3), which requires operations on
two futexes, the one used to implement the mutex and the
one used in the implementation of the wait queue associâ
ated with the condition variable. FUTEX_WAKE_OP allows
such cases to be implemented without leading to high rates
of contention and context switching.
The FUTEX_WAIT_OP operation is equivalent to executing the
following code atomically and totally ordered with respect
to other futex operations on any of the two supplied futex
words:
int oldval = *(int *) uaddr2;
*(int *) uaddr2 = oldval op oparg;
futex(uaddr, FUTEX_WAKE, val, 0, 0, 0);
if (oldval cmp cmparg)
futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0);
In other words, FUTEX_WAIT_OP does the following:
* saves the original value of the futex word at uaddr2
and performs an operation to modify the value of the
futex at uaddr2; this is an atomic read-modify-write
memory access (i.e., using atomic machine instructions
of the respective architecture)
* wakes up a maximum of val waiters on the futex for the
futex word at uaddr; and
* dependent on the results of a test of the original
value of the futex word at uaddr2, wakes up a maximum
of val2 waiters on the futex for the futex word at
uaddr2.
The operation and comparison that are to be performed are
encoded in the bits of the argument val3. Pictorially,
the encoding is:
+---+---+-----------+-----------+
|op |cmp| oparg | cmparg |
+---+---+-----------+-----------+
4 4 12 12 <== # of bits
Expressed in code, the encoding is:
#define FUTEX_OP(op, oparg, cmp, cmparg) \
(((op & 0xf) << 28) | \
((cmp & 0xf) << 24) | \
((oparg & 0xfff) << 12) | \
(cmparg & 0xfff))
In the above, op and cmp are each one of the codes listed
below. The oparg and cmparg components are literal
numeric values, except as noted below.
The op component has one of the following values:
FUTEX_OP_SET 0 /* uaddr2 = oparg; */
FUTEX_OP_ADD 1 /* uaddr2 += oparg; */
FUTEX_OP_OR 2 /* uaddr2 |= oparg; */
FUTEX_OP_ANDN 3 /* uaddr2 &= ~oparg; */
FUTEX_OP_XOR 4 /* uaddr2 ^= oparg; */
In addition, bit-wise ORing the following value into op
causes (1 << oparg) to be used as the operand:
FUTEX_OP_ARG_SHIFT 8 /* Use (1 << oparg) as operand */
The cmp field is one of the following:
FUTEX_OP_CMP_EQ 0 /* if (oldval == cmparg) wake */
FUTEX_OP_CMP_NE 1 /* if (oldval != cmparg) wake */
FUTEX_OP_CMP_LT 2 /* if (oldval < cmparg) wake */
FUTEX_OP_CMP_LE 3 /* if (oldval <= cmparg) wake */
FUTEX_OP_CMP_GT 4 /* if (oldval > cmparg) wake */
FUTEX_OP_CMP_GE 5 /* if (oldval >= cmparg) wake */
The return value of FUTEX_WAKE_OP is the sum of the number
of waiters woken on the futex uaddr plus the number of
waiters woken on the futex uaddr2.
FUTEX_WAIT_BITSET (since Linux 2.6.25)
This operation is like FUTEX_WAIT except that val3 is used
to provide a 32-bit bitset to the kernel. This bitset is
stored in the kernel-internal state of the waiter. See
the description of FUTEX_WAKE_BITSET for further details.
The FUTEX_WAIT_BITSET operation also interprets the timeâ
out argument differently from FUTEX_WAIT. See the discusâ
sion of FUTEX_CLOCK_REALTIME, above.
The uaddr2 argument is ignored.
FUTEX_WAKE_BITSET (since Linux 2.6.25)
This operation is the same as FUTEX_WAKE except that the
val3 argument is used to provide a 32-bit bitset to the
kernel. This bitset is used to select which waiters
should be woken up. The selection is done by a bit-wise
AND of the "wake" bitset (i.e., the value in val3) and the
bitset which is stored in the kernel-internal state of the
waiter (the "wait" bitset that is set using
FUTEX_WAIT_BITSET). All of the waiters for which the
result of the AND is nonzero are woken up; the remaining
waiters are left sleeping.
The effect of FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET is
to allow selective wake-ups among multiple waiters that
are blocked on the same futex. However, note that,
depending on the use case, employing this bitset multiâ
plexing feature on a futex can be less efficient than simâ
ply using multiple futexes, because employing bitset mulâ
tiplexing requires the kernel to check all waiters on a
futex, including those that are not interested in being
woken up (i.e., they do not have the relevant bit set in
their "wait" bitset).
The uaddr2 and timeout arguments are ignored.
The FUTEX_WAIT and FUTEX_WAKE operations correspond to
FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET operations where
the bitsets are all ones.
Priority-inheritance futexes
Linux supports priority-inheritance (PI) futexes in order to hanâ
dle priority-inversion problems that can be encountered with norâ
mal futex locks. Priority inversion is the problem that occurs
when a high-priority task is blocked waiting to acquire a lock
held by a low-priority task, while tasks at an intermediate priâ
ority continuously preempt the low-priority task from the CPU.
Consequently, the low-priority task makes no progress toward
releasing the lock, and the high-priority task remains blocked.
Priority inheritance is a mechanism for dealing with the priorâ
ity-inversion problem. With this mechanism, when a high-priority
task becomes blocked by a lock held by a low-priority task, the
priority of the low-priority task is temporarily raised to that
of the high-priority task, so that it is not preempted by any
intermediate level tasks, and can thus make progress toward
releasing the lock. To be effective, priority inheritance must
be transitive, meaning that if a high-priority task blocks on a
lock held by a lower-priority task that is itself blocked by a
lock held by another intermediate-priority task (and so on, for
chains of arbitrary length), then both of those tasks (or more
generally, all of the tasks in a lock chain) have their prioriâ
ties raised to be the same as the high-priority task.
From a user-space perspective, what makes a futex PI-aware is a
policy agreement (described below) between user space and the
kernel about the value of the futex word, coupled with the use of
the PI-futex operations described below. (Unlike the other futex
operations described above, the PI-futex operations are designed
for the implementation of very specific IPC mechanisms.)
The PI-futex operations described below differ from the other
futex operations in that they impose policy on the use of the
value of the futex word:
* If the lock is not acquired, the futex word's value shall be
0.
* If the lock is acquired, the futex word's value shall be the
thread ID (TID; see gettid(2)) of the owning thread.
* If the lock is owned and there are threads contending for the
lock, then the FUTEX_WAITERS bit shall be set in the futex
word's value; in other words, this value is:
FUTEX_WAITERS | TID
(Note that is invalid for a PI futex word to have no owner and
FUTEX_WAITERS set.)
With this policy in place, a user-space application can acquire
an unacquired lock or release a lock using atomic instructions
executed in user mode (e.g., a compare-and-swap operation such as
cmpxchg on the x86 architecture). Acquiring a lock simply conâ
sists of using compare-and-swap to atomically set the futex
word's value to the caller's TID if its previous value was 0.
Releasing a lock requires using compare-and-swap to set the futex
word's value to 0 if the previous value was the expected TID.
If a futex is already acquired (i.e., has a nonzero value), waitâ
ers must employ the FUTEX_LOCK_PI operation to acquire the lock.
If other threads are waiting for the lock, then the FUTEX_WAITERS
bit is set in the futex value; in this case, the lock owner must
employ the FUTEX_UNLOCK_PI operation to release the lock.
In the cases where callers are forced into the kernel (i.e.,
required to perform a futex() call), they then deal directly with
a so-called RT-mutex, a kernel locking mechanism which implements
the required priority-inheritance semantics. After the RT-mutex
is acquired, the futex value is updated accordingly, before the
calling thread returns to user space.
It is important to note that the kernel will update the futex
word's value prior to returning to user space. (This prevents
the possibility of the futex word's value ending up in an invalid
state, such as having an owner but the value being 0, or having
waiters but not having the FUTEX_WAITERS bit set.)
If a futex has an associated RT-mutex in the kernel (i.e., there
are blocked waiters) and the owner of the futex/RT-mutex dies
unexpectedly, then the kernel cleans up the RT-mutex and hands it
over to the next waiter. This in turn requires that the user-
space value is updated accordingly. To indicate that this is
required, the kernel sets the FUTEX_OWNER_DIED bit in the futex
word along with the thread ID of the new owner. User space is
then responsible for cleaning up the stale state left over by the
dead owner.
PI futexes are operated on by specifying one of the values listed
below in futex_op. Note that the PI futex operations must be
used as paired operations and are subject to some additional
requirements:
* FUTEX_LOCK_PI and FUTEX_TRYLOCK_PI pair with FUTEX_UNLOCK_PI.
FUTEX_UNLOCK_PI must be called only on a futex owned by the
calling thread, as defined by the value policy, otherwise the
error EPERM results.
* FUTEX_WAIT_REQUEUE_PI pairs with FUTEX_CMP_REQUEUE_PI. This
must be performed from a non-PI futex to a distinct PI futex
(or the error EINVAL results). Additionally, val (the number
of waiters to be woken) must be 1 (or the error EINVAL
results).
The PI futex operations are as follows:
FUTEX_LOCK_PI (since Linux 2.6.18)
This operation is used after after an attempt to acquire
the lock via an atomic user-mode instruction failed
because the futex word has a nonzero valueâspecifically,
because it contained the (PID-namespace-specific) TID of
the lock owner.
The operation checks the value of the futex word at the
address uaddr. If the value is 0, then the kernel tries
to atomically set the futex value to the caller's TID. If
the futex word's value is nonzero, the kernel atomically
sets the FUTEX_WAITERS bit, which signals the futex owner
that it cannot unlock the futex in user space atomically
by setting the futex value to 0. After that, the kernel:
1. Tries to find the thread which is associated with the
owner TID.
2. Creates or reuses kernel state on behalf of the owner.
(If this is the first waiter, there is no kernel state
for this futex, so kernel state is created by locking
the RT-mutex and the futex owner is made the owner of
the RT-mutex. If there are existing waiters, then the
existing state is reused.)
3. Attaches the waiter to the futex (i.e., the waiter is
enqueued on the RT-mutex waiter list).
If more than one waiter exists, the enqueueing of the
waiter is in descending priority order. (For information
on priority ordering, see the discussion of the
SCHED_DEADLINE, SCHED_FIFO, and SCHED_RR scheduling poliâ
cies in sched(7).) The owner inherits either the waiter's
CPU bandwidth (if the waiter is scheduled under the
SCHED_DEADLINE policy) or the waiter's priority (if the
waiter is scheduled under the SCHED_RR or SCHED_FIFO polâ
icy). This inheritance follows the lock chain in the case
of nested locking and performs deadlock detection.
The timeout argument provides a timeout for the lock
attempt. It is interpreted as an absolute time, measured
against the CLOCK_REALTIME clock. If timeout is NULL, the
operation will block indefinitely.
The uaddr2, val, and val3 arguments are ignored.
FUTEX_TRYLOCK_PI (since Linux 2.6.18)
This operation tries to acquire the futex at uaddr. It is
invoked when a user-space atomic acquire did not succeed
because the futex word was not 0.
FIXME(Next sentence) The wording "The trylock in kernel" below
needs clarification. Suggestions?
The trylock in kernel might succeed because the futex word
contains stale state (FUTEX_WAITERS and/or
FUTEX_OWNER_DIED). This can happen when the owner of the
futex died. User space cannot handle this condition in a
race-free manner, but the kernel can fix this up and
acquire the futex.
The uaddr2, val, timeout, and val3 arguments are ignored.
FUTEX_UNLOCK_PI (since Linux 2.6.18)
This operation wakes the top priority waiter that is waitâ
ing in FUTEX_LOCK_PI on the futex address provided by the
uaddr argument.
This is called when the user-space value at uaddr cannot
be changed atomically from a TID (of the owner) to 0.
The uaddr2, val, timeout, and val3 arguments are ignored.
FUTEX_CMP_REQUEUE_PI (since Linux 2.6.31)
This operation is a PI-aware variant of FUTEX_CMP_REQUEUE.
It requeues waiters that are blocked via
FUTEX_WAIT_REQUEUE_PI on uaddr from a non-PI source futex
(uaddr) to a PI target futex (uaddr2).
As with FUTEX_CMP_REQUEUE, this operation wakes up a maxiâ
mum of val waiters that are waiting on the futex at uaddr.
However, for FUTEX_CMP_REQUEUE_PI, val is required to be 1
(since the main point is to avoid a thundering herd). The
remaining waiters are removed from the wait queue of the
source futex at uaddr and added to the wait queue of the
target futex at uaddr2.
The val2 and val3 arguments serve the same purposes as for
FUTEX_CMP_REQUEUE.
FUTEX_WAIT_REQUEUE_PI (since Linux 2.6.31)
Wait on a non-PI futex at uaddr and potentially be
requeued (via a FUTEX_CMP_REQUEUE_PI operation in another
task) onto a PI futex at uaddr2. The wait operation on
uaddr is the same as for FUTEX_WAIT.
The waiter can be removed from the wait on uaddr without
requeueing on uaddr2 via a FUTEX_WAKE operation in another
task. In this case, the FUTEX_WAIT_REQUEUE_PI operation
returns with the error EWOULDBLOCK.
If timeout is not NULL, it specifies a timeout for the
wait operation; this timeout is interpreted as outlined
above in the description of the FUTEX_CLOCK_REALTIME
option. If timeout is NULL, the operation can block
indefinitely.
The val3 argument is ignored.
The FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI were
added to support a fairly specific use case: support for
priority-inheritance-aware POSIX threads condition variâ
ables. The idea is that these operations should always be
paired, in order to ensure that user space and the kernel
remain in sync. Thus, in the FUTEX_WAIT_REQUEUE_PI operaâ
tion, the user-space application pre-specifies the target
of the requeue that takes place in the
FUTEX_CMP_REQUEUE_PI operation.
RETURN VALUE
In the event of an error (and assuming that futex() was invoked
via syscall(2)), all operations return -1 and set errno to indiâ
cate the cause of the error.
The return value on success depends on the operation, as
described in the following list:
FUTEX_WAIT
Returns 0 if the caller was woken up. Note that a wake-up
can also be caused by common futex usage patterns in unreâ
lated code that happened to have previously used the futex
word's memory location (e.g., typical futex-based impleâ
mentations of Pthreads mutexes can cause this under some
conditions). Therefore, callers should always conservaâ
tively assume that a return value of 0 can mean a spurious
wake-up, and use the futex word's value (i.e., the user
space synchronization scheme)
to decide whether to continue to block or not.
FUTEX_WAKE
Returns the number of waiters that were woken up.
FUTEX_FD
Returns the new file descriptor associated with the futex.
FUTEX_REQUEUE
Returns the number of waiters that were woken up.
FUTEX_CMP_REQUEUE
Returns the total number of waiters that were woken up or
requeued to the futex for the futex word at uaddr2. If
this value is greater than val, then the difference is the
number of waiters requeued to the futex for the futex word
at uaddr2.
FUTEX_WAKE_OP
Returns the total number of waiters that were woken up.
This is the sum of the woken waiters on the two futexes
for the futex words at uaddr and uaddr2.
FUTEX_WAIT_BITSET
Returns 0 if the caller was woken up. See FUTEX_WAIT for
how to interpret this correctly in practice.
FUTEX_WAKE_BITSET
Returns the number of waiters that were woken up.
FUTEX_LOCK_PI
Returns 0 if the futex was successfully locked.
FUTEX_TRYLOCK_PI
Returns 0 if the futex was successfully locked.
FUTEX_UNLOCK_PI
Returns 0 if the futex was successfully unlocked.
FUTEX_CMP_REQUEUE_PI
Returns the total number of waiters that were woken up or
requeued to the futex for the futex word at uaddr2. If
this value is greater than val, then difference is the
number of waiters requeued to the futex for the futex word
at uaddr2.
FUTEX_WAIT_REQUEUE_PI
Returns 0 if the caller was successfully requeued to the
futex for the futex word at uaddr2.
ERRORS
EACCES No read access to the memory of a futex word.
EAGAIN (FUTEX_WAIT, FUTEX_WAIT_BITSET, FUTEX_WAIT_REQUEUE_PI) The
value pointed to by uaddr was not equal to the expected
value val at the time of the call.
Note: on Linux, the symbolic names EAGAIN and EWOULDBLOCK
(both of which appear in different parts of the kernel
futex code) have the same value.
EAGAIN (FUTEX_CMP_REQUEUE, FUTEX_CMP_REQUEUE_PI) The value
pointed to by uaddr is not equal to the expected value
val3.
EAGAIN (FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_CMP_REQUEUE_PI)
The futex owner thread ID of uaddr (for
FUTEX_CMP_REQUEUE_PI: uaddr2) is about to exit, but has
not yet handled the internal state cleanup. Try again.
EDEADLK
(FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_CMP_REQUEUE_PI)
The futex word at uaddr is already locked by the caller.
EDEADLK
(FUTEX_CMP_REQUEUE_PI) While requeueing a waiter to the PI
futex for the futex word at uaddr2, the kernel detected a
deadlock.
EFAULT A required pointer argument (i.e., uaddr, uaddr2, or timeâ
out) did not point to a valid user-space address.
EINTR A FUTEX_WAIT or FUTEX_WAIT_BITSET operation was interâ
rupted by a signal (see signal(7)). In kernels before
Linux 2.6.22, this error could also be returned for on a
spurious wakeup; since Linux 2.6.22, this no longer hapâ
pens.
EINVAL The operation in futex_op is one of those that employs a
timeout, but the supplied timeout argument was invalid
(tv_sec was less than zero, or tv_nsec was not less than
1,000,000,000).
EINVAL The operation specified in futex_op employs one or both of
the pointers uaddr and uaddr2, but one of these does not
point to a valid objectâthat is, the address is not four-
byte-aligned.
EINVAL (FUTEX_WAIT_BITSET, FUTEX_WAKE_BITSET) The bitset supplied
in val3 is zero.
EINVAL (FUTEX_CMP_REQUEUE_PI) uaddr equals uaddr2 (i.e., an
attempt was made to requeue to the same futex).
EINVAL (FUTEX_FD) The signal number supplied in val is invalid.
EINVAL (FUTEX_WAKE, FUTEX_WAKE_OP, FUTEX_WAKE_BITSET,
FUTEX_REQUEUE, FUTEX_CMP_REQUEUE) The kernel detected an
inconsistency between the user-space state at uaddr and
the kernel stateâthat is, it detected a waiter which waits
in FUTEX_LOCK_PI on uaddr.
EINVAL (FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_UNLOCK_PI) The
kernel detected an inconsistency between the user-space
state at uaddr and the kernel state. This indicates
either state corruption or that the kernel found a waiter
on uaddr which is waiting via FUTEX_WAIT or
FUTEX_WAIT_BITSET.
EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an inconsisâ
tency between the user-space state at uaddr2 and the kerâ
nel state; that is, the kernel detected a waiter which
waits via FUTEX_WAIT or FUTEX_WAIT_BITSET on uaddr2.
EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an inconsisâ
tency between the user-space state at uaddr and the kernel
state; that is, the kernel detected a waiter which waits
via FUTEX_WAIT or FUTEX_WAIT_BITESET on uaddr.
EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an inconsisâ
tency between the user-space state at uaddr and the kernel
state; that is, the kernel detected a waiter which waits
on uaddr via FUTEX_LOCK_PI (instead of
FUTEX_WAIT_REQUEUE_PI).
EINVAL (FUTEX_CMP_REQUEUE_PI) An attempt was made to requeue a
waiter to a futex other than that specified by the matchâ
ing FUTEX_WAIT_REQUEUE_PI call for that waiter.
EINVAL (FUTEX_CMP_REQUEUE_PI) The val argument is not 1.
EINVAL Invalid argument.
ENOMEM (FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_CMP_REQUEUE_PI)
The kernel could not allocate memory to hold state inforâ
mation.
ENFILE (FUTEX_FD) The system limit on the total number of open
files has been reached.
ENOSYS Invalid operation specified in futex_op.
ENOSYS The FUTEX_CLOCK_REALTIME option was specified in futex_op,
but the accompanying operation was neither FUTEX_WAIT_BITâ
SET nor FUTEX_WAIT_REQUEUE_PI.
ENOSYS (FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_UNLOCK_PI,
FUTEX_CMP_REQUEUE_PI, FUTEX_WAIT_REQUEUE_PI) A run-time
check determined that the operation is not available. The
PI-futex operations are not implemented on all architecâ
tures and are not supported on some CPU variants.
EPERM (FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_CMP_REQUEUE_PI)
The caller is not allowed to attach itself to the futex at
uaddr (for FUTEX_CMP_REQUEUE_PI: the futex at uaddr2).
(This may be caused by a state corruption in user space.)
EPERM (FUTEX_UNLOCK_PI) The caller does not own the lock repreâ
sented by the futex word.
ESRCH (FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_CMP_REQUEUE_PI)
The thread ID in the futex word at uaddr does not exist.
ESRCH (FUTEX_CMP_REQUEUE_PI) The thread ID in the futex word at
uaddr2 does not exist.
ETIMEDOUT
The operation in futex_op employed the timeout specified
in timeout, and the timeout expired before the operation
completed.
VERSIONS
Futexes were first made available in a stable kernel release with
Linux 2.6.0.
Initial futex support was merged in Linux 2.5.7 but with differâ
ent semantics from what was described above. A four-argument
system call with the semantics described in this page was introâ
duced in Linux 2.5.40. A fifth argument was added in Linux
2.5.70, and a sixth argument was added in Linux 2.6.7.
CONFORMING TO
This system call is Linux-specific.
NOTES
Glibc does not provide a wrapper for this system call; call it
using syscall(2).
Several higher-level programming abstractions are implemented via
futexes, including POSIX semaphores and various POSIX threads
synchronization mechanisms (mutexes, condition variables, read-
write locks, and barriers).
EXAMPLE
FIXME I think it would be helpful here to say a few more words about
the difference(s) between FUTEX_LOCK_PI and FUTEX_TRYLOCK_PI.
Can someone propose something?
The program below demonstrates use of futexes in a program where
parent and child use a pair of futexes located inside a shared
anonymous mapping to synchronize access to a shared resource: the
terminal. The two processes each write nloops (a command-line
argument that defaults to 5 if omitted) messages to the terminal
and employ a synchronization protocol that ensures that they
alternate in writing messages. Upon running this program we see
output such as the following:
$ ./futex_demo
Parent (18534) 0
Child (18535) 0
Parent (18534) 1
Child (18535) 1
Parent (18534) 2
Child (18535) 2
Parent (18534) 3
Child (18535) 3
Parent (18534) 4
Child (18535) 4
Program source
/* futex_demo.c
Usage: futex_demo [nloops]
(Default: 5)
Demonstrate the use of futexes in a program where parent and child
use a pair of futexes located inside a shared anonymous mapping to
synchronize access to a shared resource: the terminal. The two
processes each write 'num-loops' messages to the terminal and employ
a synchronization protocol that ensures that they alternate in
writing messages.
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <sys/time.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
static int *futex1, *futex2, *iaddr;
static int
futex(int *uaddr, int futex_op, int val,
const struct timespec *timeout, int *uaddr2, int val3)
{
return syscall(SYS_futex, uaddr, futex_op, val,
timeout, uaddr, val3);
}
/* Acquire the futex pointed to by 'futexp': wait for its value to
become 1, and then set the value to 0. */
static void
fwait(int *futexp)
{
int s;
/* __sync_bool_compare_and_swap(ptr, oldval, newval) is a gcc
built-in function. It atomically performs the equivalent of:
if (*ptr == oldval)
*ptr = newval;
It returns true if the test yielded true and *ptr was updated.
The alternative here would be to employ the equivalent atomic
machine-language instructions. For further information, see
the GCC Manual. */
while (1) {
/* Is the futex available? */
if (__sync_bool_compare_and_swap(futexp, 1, 0))
break; /* Yes */
/* Futex is not available; wait */
s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
if (s == -1 && errno != EAGAIN)
errExit("futex-FUTEX_WAIT");
}
}
/* Release the futex pointed to by 'futexp': if the futex currently
has the value 0, set its value to 1 and the wake any futex waiters,
so that if the peer is blocked in fpost(), it can proceed. */
static void
fpost(int *futexp)
{
int s;
/* __sync_bool_compare_and_swap() was described in comments above */
if (__sync_bool_compare_and_swap(futexp, 0, 1)) {
s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
if (s == -1)
errExit("futex-FUTEX_WAKE");
}
}
int
main(int argc, char *argv[])
{
pid_t childPid;
int j, nloops;
setbuf(stdout, NULL);
nloops = (argc > 1) ? atoi(argv[1]) : 5;
/* Create a shared anonymous mapping that will hold the futexes.
Since the futexes are being shared between processes, we
subsequently use the "shared" futex operations (i.e., not the
ones suffixed "_PRIVATE") */
iaddr = mmap(NULL, sizeof(int) * 2, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_SHARED, -1, 0);
if (iaddr == MAP_FAILED)
errExit("mmap");
futex1 = &iaddr[0];
futex2 = &iaddr[1];
*futex1 = 0; /* State: unavailable */
*futex2 = 1; /* State: available */
/* Create a child process that inherits the shared anonymous
mapping */
childPid = fork();
if (childPid == -1)
errExit("fork");
if (childPid == 0) { /* Child */
for (j = 0; j < nloops; j++) {
fwait(futex1);
printf("Child (%ld) %d\n", (long) getpid(), j);
fpost(futex2);
}
exit(EXIT_SUCCESS);
}
/* Parent falls through to here */
for (j = 0; j < nloops; j++) {
fwait(futex2);
printf("Parent (%ld) %d\n", (long) getpid(), j);
fpost(futex1);
}
wait(NULL);
exit(EXIT_SUCCESS);
}
SEE ALSO
get_robust_list(2), restart_syscall(2), pthread_mutexattr_getproâ
tocol(3), futex(7), sched(7)
The following kernel source files:
* Documentation/pi-futex.txt
* Documentation/futex-requeue-pi.txt
* Documentation/locking/rt-mutex.txt
* Documentation/locking/rt-mutex-design.txt
* Documentation/robust-futex-ABI.txt
Franke, H., Russell, R., and Kirwood, M., 2002. Fuss, Futexes
and Furwocks: Fast Userlevel Locking in Linux (from proceedings
of the Ottawa Linux Symposium 2002),
âhttp://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdfâ;
Hart, D., 2009. A futex overview and update,
âhttp://lwn.net/Articles/360699/â;
Hart, D. and Guniguntala, D., 2009. Requeue-PI: Making Glibc
Condvars PI-Aware (from proceedings of the 2009 Real-Time Linux
Workshop),
âhttp://lwn.net/images/conf/rtlws11/papers/proc/p10.pdfâ;
Drepper, U., 2011. Futexes Are Tricky,
âhttp://www.akkadia.org/drepper/futex.pdfâ;
Futex example library, futex-*.tar.bz2 at
âftp://ftp.kernel.org/pub/linux/kernel/people/rusty/â;
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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