futex(3) man page, final draft for pre-release review

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 block‐ ing construct in the context of shared-memory synchronization. When using futexes, the majority of the synchronization opera‐
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 value—referred to below as a futex word—whose 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 load‐ ing of the futex word's value, the comparison of that value with the expected value, and the actual blocking will happen atomi‐

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 imple‐ 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 oper‐ 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 represent‐ 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 unnec‐ 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 speci‐
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 sec‐ ond futex word that is employed by the operation. The interpre‐ tation of the final integer argument, val3, depends on the opera‐ tion.

Futex operations
The futex_op argument consists of two parts: a command that spec‐ 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 con‐ 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 archi‐ 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 indef‐ 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 wait‐ 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 associ‐ 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 archi‐
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. (Speci‐
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 wait‐ 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 associ‐
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 time‐ out argument differently from FUTEX_WAIT. See the discus‐ 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 multi‐ plexing feature on a futex can be less efficient than sim‐ ply using multiple futexes, because employing bitset mul‐ 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 han‐ dle priority-inversion problems that can be encountered with nor‐ 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 pri‐ 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 prior‐ 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 priori‐ 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 con‐ 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), wait‐ 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 value—specifically, 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 poli‐ 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 pol‐ 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 wait‐ 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 maxi‐ 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 vari‐ 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 opera‐ 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 indi‐ 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 unre‐ lated code that happened to have previously used the futex word's memory location (e.g., typical futex-based imple‐ mentations of Pthreads mutexes can cause this under some
conditions). Therefore, callers should always conserva‐
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 time‐
out) did not point to a valid user-space address.

EINTR A FUTEX_WAIT or FUTEX_WAIT_BITSET operation was inter‐
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 hap‐ 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 object—that 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 state—that 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 inconsis‐
tency between the user-space state at uaddr2 and the ker‐ 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 inconsis‐
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 inconsis‐
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 match‐ 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 infor‐ 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_BIT‐ 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 architec‐ 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 repre‐
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 differ‐ ent semantics from what was described above. A four-argument system call with the semantics described in this page was intro‐
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_getpro‐ 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), ⟨http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf⟩

Hart, D., 2009. A futex overview and update, ⟨http://lwn.net/Articles/360699/⟩

Hart, D. and Guniguntala, D., 2009. Requeue-PI: Making Glibc Condvars PI-Aware (from proceedings of the 2009 Real-Time Linux Workshop), ⟨http://lwn.net/images/conf/rtlws11/papers/proc/p10.pdf⟩

Drepper, U., 2011. Futexes Are Tricky,
⟨http://www.akkadia.org/drepper/futex.pdf⟩

Futex example library, futex-*.tar.bz2 at ⟨ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/⟩

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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This looks good to me; I just saw minor things (see below). Thank you for all the work you put into this (and to everybody who contributed)!

Maybe -- should we just say that it refers to the mathematical notion of a total order (or, technically, a strict total order in this case)? Though I would hope that everyone using futexes is roughly aware of the differences between partial and total orders.

s/futex/lock/ to make it consistent with FUTEX_LOCK.

What about "The acquisition of the lock might suceed if performed by the kernel in cases when the futex word contains stale state...".

This should be EAGAIN, I suppose, or the enumeration of errors should include EWOULDBLOCK.

Torvald

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Hello Torvald,

Hey Torvald, you were one of the biggest contributors, so, thanks!

I added a sentence along those lines.

Done.

Sounds good to me. Changed.

Changed. BTW, under ERRORS there is already this text:

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.

Thanks for the comments, Torvald!

Cheers,

Michael

Michael Kerrisk
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Fantastic! A few comments below.

...

Totally ordered with respect futex operations refers to semantics of the ACQUIRE/RELEASE operations and how they impact ordering of memory reads and writes. The kernel futex operations are protected by spinlocks, which ensure that that all operations are serialized with respect to one another.

This is a lot to attempt to define in this document. Perhaps a reference to linux/Documentation/memory-barriers.txt as a footnote would be sufficient? Or perhaps for this manual, "serialized" would be sufficient, with a footnote regarding "totally ordered" and a pointer to the memory-barrier documentation?

...

...

That caught me by surprise, but it's true. We reject FUTEX_WAIT | FUTEX_CLOCK_REALTIME, even though FUTEX_WAIT treated as FUTEX_WAIT_BITSET with val3=FUTEX_BITSET_MATCH_ANY.

Thomas, this looks like an oversight to me - do you recall if we intentionally disallow FUTEX_CLOCK_REALTIME with FUTEX_WAIT?

...

...

^ it

...

The lock acquisition might succeed in the kernel because the futex word

...

Hrm. It seems pretty straightforward to me. I guess I'm too close to it. What about it seems unclear and needs clarification?

Darren Hart
Intel Open Source Technology Center
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Hello Darren,

[...]

I think I'll just settle for writing serialized in the man page, and be done with it :-).

You uncover all sorts of interesting stuff when you document APIs ;-).

Already did some rewording here which I think makes things better.

On reflection, I agree that the difference is perhaps well-enough explained.

Thanks for the comments, Darren.

Cheers,

Michael

Michael Kerrisk
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I'd prefer if you'd not just use "serialized" :) Eventually, I'd prefer if we can explain the semantics for the user in terms of the terminology and semantics of the memory model of the programming language that users will likely use to call futex ops (ie, C11 / C++11).

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FWIW a couple of uses of "serialized" were replaced in the C++11 final draft due to comments pointing out that term is not defined in the standard, see http://wg21.link/lwg1494 and http://wg21.link/lwg1504

That's not quite the same, because an ISO standard is supposed to define all terms it uses, even for something like "serialized" where the meaning is commonly understood by those in the field.

But I do like Torvald's suggestion to describe the semantics in similar terms to C11, because that's the user-space model that non-kernel folks (like me) are more likely to be familiar with.

Overall I like the new page a lot, I found it clear and readable. Nice work.

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Sigh :-). Okay--removed.

And I'd be really happy to see such an explanation land in the page.

Cheers,

Michael

Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

I'd strongly prefer to document the semantics for users here. And I don't think users use the kernel's memory model -- instead, if we assume that most users will call futex ops from C or C++, then the best we have is the C11 / C++11 memory model. Therefore, if we want to expand that, we should specify semantics in terms of as-if equivalence to C11 pseudo code. I had proposed that in the past but, IIRC, Michael didn't want to add a C11 "dependency" in the semantics back then, at least for the initial release.

Here's what I wrote back then (atomic_*_relaxed() is like C11 atomic_*(..., memory_order_relaxed), lock/unlock have normal C11 mutex semantics):

========================

For example, we could say that futex_wait is, in terms of synchronization semantics, *as if* we'd execute a piece of C11 code. Here's a part of the docs for a glibc-internal futex wrapper that I'm working on; this is futex_wait ... :

/* Atomically wrt other futex operations, this blocks iff the value at
*FUTEX matches the expected value. This is semantically equivalent to:

     l = <get lock associated with futex> (FUTEX);
     wait_flag = <get wait_flag associated with futex> (FUTEX);
     lock (l);
     val = atomic_load_relaxed (FUTEX);
     if (val != expected) { unlock (l); return EAGAIN; }
     atomic_store_relaxed (wait_flag, 1);
     unlock (l);
     // Now block; can time out in futex_time_wait (see below)
     while (atomic_load_relaxed(wait_flag));

Note that no guarantee of a happens-before relation between a woken futex_wait and a futex_wake is documented; however, this does not matter in practice because we have to consider spurious wake-ups (see below), and thus would not be able to reason which futex_wake woke us anyway.

... and this is futex_wake:

/* Atomically wrt other futex operations, this unblocks the specified
number of processes, or all processes blocked on this futex if there are fewer than the specified number. Semantically, this is equivalent to:

     l = <get lock associated with futex> (futex);
     lock (l);
     for (res = 0; processes_to_wake > 0; processes_to_wake--, res++) {
       if (<no process blocked on futex>) break;
       wf = <get wait_flag of a process blocked on futex> (futex);
       // No happens-before guarantee with woken futex_wait (see above)
       atomic_store_relaxed (wf, 0);
     }
     return res;

This allows a programmer to really infer the guarantees he/she can get from a futex in terms of synchronization, without the docs having to use prose to describe that. This should also not constrain the kernel in terms of how to implement it, because it is a conceptual as-if relation (e.g., the kernel won't spin-wait the whole time, and we might want to make this clear for the PI case).

Of course, there are several as-if representations we could use, and we might want to be a bit more pseudo-code-ish to make this also easy to understand for people not familiar with C11 (e.g., using mutex + condvar with some relaxation of condvar guaranteees).

=========================

I will go through the discussion pointed out by Davidlohr next.

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Yes, please.

Agreed.

I think we should. And by we, I mean you ;-)

I'd like to avoid it if possible, since many of us don't understand all the details of those C11 semantics--and by us, I mean me :-/. But maybe I'll be forced to educate myself better.

Okay -- I'm open to all of the above.

Cheers,

Michael

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So there are two things here regarding ordering. One is the most obvious which is ordered due to the taking/dropping the hb spinlock. Secondly, its the cases which Peter brought up a while ago that involves atomic futex ops
futex_atomic_*(), which do not have clearly defined semantics, and you get
inconsistencies with certain archs (tile being the worst iirc).

But anyway, the important thing users need to know about is that the atomic futex operation must be totally ordered wrt any other user tasks that are trying to access that address. This is not necessarily the case for kernel ops. Peter illustrates this nicely with lock stealing example; (see https://lkml.org/lkml/2015/8/26/596).

Internally, I believe we decided that making it fully ordered (as opposed to making use of implicit barriers for ACQUIRE/RELEASE), so you'd endup having an MB ll/sc MB kind of setup.

[...]

Nit, but for this we have err(3).

Not related, but it looks like we should have a Documentation/futex/ folder here.

Thanks,
Davidlohr
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Hi David,

Thanks. I reworded things here a little.

I don't much like them though (not in POSIX).

Thanks for the help David.

Cheers,

Michael

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I suppose that this means what is described in the manpage already? That is, that futex operations (ie, the syscalls) are atomic wrt each other and in a strict total order?

OK. So, from a user's POV, this is about the semantics of the kernel's accesses to the futex word. I agree that specifying this more clearly would be helpful.

First, there are the comparisons of the futex words used in, for example, FUTEX_WAIT. They should use an atomic load within the conceptual critical sections that make up futex operations. This load itself doesn't need to establish any ordering, so it can be equivalent to a C11 memory_order_relaxed load. Are there any objections to that?

Second, We have the write accesses in FUTEX_[TRY]LOCK_PI and FUTEX_UNLOCK_PI. We already specify those as atomic and within the conceptual critical sections of the futex operation. In addition, they should establish ordering themselves, so C11 have memory_order_acquire / memory_order_release semantics. Specifying this would be good. Any objections to these semantics?

Third, we have the atomic read-modify-write operation that is part of FUTEX_WAKE_OP (ie, AFAIU, the case you pointed at specifically). I don't have a strong opinion on what it should be, because I think userspace can enforce the orderings it needs on its own (eg, if I interpret Peter Zijlstra's example correctly, userspace can add appropriate fences before the CPU0/futex_unlock and after the CPU2/futex_load calls). FUTEX_WAKE_OP accesses no other userspace memory location, so there's no ordering relation to other accesses to userspace memory that userspace cannot affect.
OTOH, legacy userspace may have assumed strong semantics, so making the read-modify-write have memory_order_seq_cst semantics is probably a safe bet. Futex operations typically shouldn't be on the fast paths anyway.

I'm not sure what you mean precisely. One can't order the whole futex operations totally wrt memory accesses by userspace because they'd need to synchronize to do that, and thus userspace would to hvae either hook into the kernel's synchronization or use HTM or such.

OK. So, any objections to documenting that the read-modify-write op in FUTEX_WAKE_OP has memory_order_seq_cst semantics?

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