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Date: Sat, 28 Mar 2015 12:47:25 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Darren Hart <dvhart@...ux.intel.com>,
Carlos O'Donell <carlos@...hat.com>,
Ingo Molnar <mingo@...e.hu>, Jakub Jelinek <jakub@...hat.com>,
"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
lkml <linux-kernel@...r.kernel.org>,
Davidlohr Bueso <dave@...olabs.net>,
Arnd Bergmann <arnd@...db.de>,
Steven Rostedt <rostedt@...dmis.org>,
Linux API <linux-api@...r.kernel.org>,
Torvald Riegel <triegel@...hat.com>,
Roland McGrath <roland@...k.frob.com>,
Darren Hart <dvhart@...radead.org>,
Anton Blanchard <anton@...ba.org>,
Eric Dumazet <edumazet@...gle.com>,
bill o gallmeister <bgallmeister@...il.com>,
Jan Kiszka <jan.kiszka@...mens.com>,
Daniel Wagner <wagi@...om.org>, Rich Felker <dalias@...c.org>,
Andy Lutomirski <luto@...capital.net>,
bert hubert <bert.hubert@...herlabs.nl>,
Rusty Russell <rusty@...tcorp.com.au>,
Heinrich Schuchardt <xypron.glpk@....de>
Subject: Re: Revised futex(2) man page for review
On Sat, Mar 28, 2015 at 09:53:21AM +0100, Michael Kerrisk (man-pages) wrote:
> So, please take a look at the page below. At this point,
> I would most especially appreciate help with the FIXMEs.
For people who cannot read that troff gibberish (me)..
---
FUTEX(2) Linux Programmer's Manual FUTEX(2)
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: u32 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 blocking construct
in the context of shared-memory synchronization: The program implements
the majority of the synchronization in user space, and uses one of
operations of the system call when it is likely that it has to block
for a longer time until the condition becomes true. The program uses
another operation of the system call to wake anyone waiting for a par‐
ticular condition.
The condition is represented by the futex word, which is an address in
memory supplied to the futex() system call, and the value at this mem‐
ory location. (While the virtual addresses for the same memory in sep‐
arate processes may not be equal, the kernel maps them internally so
that the same memory mapped in different locations will correspond for
futex() calls.)
When executing a futex operation that requests to block a thread, the
kernel will only block if the futex word has the value that the calling
thread supplied as expected value. The load from the futex word, the
comparison with the expected value, and the actual blocking will happen
atomically and totally ordered with respect to concurrently executing
futex operations on the same futex word, such as operations that wake
threads blocked on this futex word. Thus, the futex word is used to
connect the synchronization in user spac with the implementation of
blocking by the kernel; similar to an atomic compare-and-exchange oper‐
ation that potentially changes shared memory, blocking via a futex is
an atomic compare-and-block operation. See NOTES for a detailed speci‐
fication of the synchronization semantics.
One example use of futexes is implementing locks. The state of the
lock (i.e., acquired or not acquired) can be represented as an atomi‐
cally 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. If a thread cannot acquire
a lock because it is already acquired by another thread, it can request
to block if and only the lock is still acquired by using the lock's
flag as futex word and expecting a value that represents the acquired
state. When releasing the lock, a thread has to first reset the lock
state to not acquired and then execute the futex operation that wakes
one thread blocked on the futex word that is the lock's flag (this can
be be further optimized to avoid unnecessary wake-ups). See futex(7)
for more detail on how to use futexes.
Besides the basic wait and wake-up futex functionality, there are fur‐
ther futex operations aimed at supporting more complex use cases. Also
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 specified 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. How‐
ever, notwithstanding the prototype shown above, for some operations,
this argument is instead a four-byte integer whose meaning is deter‐
mined by the operation. For these operations, the kernel casts the
timeout value to u32, 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 second
futex word that is employed by the operation. The interpretation of
the final integer argument, val3, depends on the operation.
Futex operations
The futex_op argument consists of two parts: a command that specifies
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 only being used for
synchronization between threads of the same process). This
allows the kernel to choose the fast path for validating the
user-space address and avoids expensive VMA lookups, taking ref‐
erence counts on file backing store, and so on.
As a convenience, <linux/futex.h> defines a set of constants
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 awaiting FUTEX_WAKE 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
architecture). 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 consid‐
ered 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 pre‐
vent 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 rel‐
ative timeout for the wait, measured according to the
CLOCK_MONOTONIC clock. (This interval will be rounded up to the
system clock granularity, and kernel scheduling delays mean that
the blocking interval may overrun by a small amount.) If time‐
out is NULL, the call blocks indefinitely.
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 waiters). 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 associated 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 notifica‐
tions: 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.
To prevent race conditions, the caller should test if the futex
has been upped after FUTEX_FD returns.
Because it was inherently racy, FUTEX_FD has been removed from
Linux 2.6.26 onward.
FUTEX_REQUEUE (since Linux 2.6.0)
Avoid using this operation. It is broken for its intended pur‐
pose. Use FUTEX_CMP_REQUEUE instead.
This operation performs the same task as FUTEX_CMP_REQUEUE,
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 architecture).
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.
This 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 only happens under
certain conditions. Both operations can be used to avoid a
"thundering herd" effect when FUTEX_WAKE is used and all of the
waiters that are woken need to acquire another futex.
Typical values to specify for val are 0 or or 1. (Specifying
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.)
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_sig‐
nal(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 associated with the condition variable.
FUTEX_WAKE_OP allows such cases to be implemented without lead‐
ing to high rates of contention and context switching.
The FUTEX_WAIT_OP operation is equivalent to execute the follow‐
ing 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 per‐
forms 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 timeout
argument differently from FUTEX_WAIT. See the discussion 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. Note, however, that using this bitset multi‐
plexing feature on a futex is less efficient than simply using
multiple futexes, because employing bitset multiplexing 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 handle
priority-inversion problems that can be encountered with normal futex
locks. Priority inversion is the problem that occurs when a high-pri‐
ority task is blocked waiting to acquire a lock held by a low-priority
task, while tasks at an intermediate priority 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 priority-
inversion problem. With this mechanism, when a high-priority task
becomes blocked by a lock held by a low-priority task, the latter's
priority is temporarily raised to that of the former, 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 lock held by
another intermediate-priority task (and so on, for chains of arbitrary
length), then both of those task (or more generally, all of the tasks
in a lock chain) have their priorities 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 between user space and the kernel about the value of the
futex word (described in a moment), coupled with the use of the PI
futex operations described below (in particular, FUTEX_LOCK_PI,
FUTEX_TRYLOCK_PI, and FUTEX_CMP_REQUEUE_PI).
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 a PI futex word never just has the value FUTEX_WAITERS, which
is a permissible state for non-PI futexes.
With this policy in place, a user-space application can acquire a not-
acquired lock or release a lock that no other threads try to acquire
using atomic instructions executed in user space (e.g., a compare-and-
swap operation such as cmpxchg on the x86 architecture). Acquiring a
lock simply consists 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), waiters
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() operation), 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. Unlike the other futex opera‐
tions described above, the PI futex operations are designed for the
implementation of very specific IPC mechanisms.
PI futexes are operated on by specifying one of the following values in
futex_op:
FUTEX_LOCK_PI (since Linux 2.6.18)
This operation is used after after an attempt to acquire the
lock via an atomic user-space instruction failed because the
futex word has a nonzero value—specifically, because it con‐
tained the 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 that fails, or 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 tries to find the thread
which is associated with the owner TID, creates or reuses kernel
state on behalf of the owner and attaches the waiter to it. The
enqueueing of the waiter is in descending priority order if more
than one waiter exists. The owner inherits either the priority
or the bandwidth of the waiter. 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 deals
with the situation where the TID value at uaddr is 0, but the
FUTEX_WAITERS bit is set. User space cannot handle this condi‐
tion in a race-free manner
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 waiting in
FUTEX_LOCK_PI on the futex address provided by the uaddr argu‐
ment.
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 maximum 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 wait‐
ers 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 operation to wait on a non-PI futex at uaddr and poten‐
tially be requeued onto a PI futex at uaddr2. The wait opera‐
tion on uaddr is the same as FUTEX_WAIT. The waiter can be
removed from the wait on uaddr via FUTEX_WAKE without requeueing
on uaddr2.
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-inheri‐
tance-aware POSIX threads condition variables. 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 operation, 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, all operations return -1 and set errno to
indicate 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 unrelated code
that happened to have previously used the futex word's memory
location (e.g., typical futex-based implementations of Pthreads
mutexes can cause this under some conditions). Therefore, call‐
ers should always conservatively 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 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. (This proba‐
bly indicates a race; use the safe FUTEX_WAKE now.)
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 timeout)
did not point to a valid user-space address.
EINTR A FUTEX_WAIT or FUTEX_WAIT_BITSET operation was interrupted 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 happens.
EINVAL The operation in futex_op is one of those that employs a time‐
out, but the supplied timeout argument was invalid (tv_sec was
less than zero, or tv_nsec was not less than 1000,000,000).
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 inconsistency
between the user-space state at uaddr2 and the kernel state;
that is, the kernel detected a waiter which waits via FUTEX_WAIT
on uaddr2.
EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an inconsistency
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 inconsistency
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 matching
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 ker‐
nel could not allocate memory to hold state information.
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_BITSET 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 architectures 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 represented
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 different
semantics from what was described above. A four-argument system call
with the semantics described in this page was introduced in Linux
2.5.40. In Linux 2.5.70, one argument was added. In Linux 2.6.7, a
sixth argument was added—messy, especially on the s390 architecture.
CONFORMING TO
This system call is Linux-specific.
NOTES
Glibc does not provide a wrapper for this system call; call it using
syscall(2).
EXAMPLE
The program below demonstrates use of futexes in a program where parent
and child use a pair of futexes located inside a shared anonymous map‐
ping 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 pro‐
tocol that ensures that they alternate in writing messages. Upon run‐
ning 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), futex(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 Fur‐
wocks: 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/⟩
Linux 2014-05-21 FUTEX(2)
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