Hello all, As becomes quickly obvious upon reading it, the current futex(2) man page is in a sorry state, lacking many important details, and also the various additions that have been made to the interface over the last years. I've been working on revising it, first of all based on input I got in response to a request for help last year (http://thread.gmane.org/gmane.linux.kernel/1703405), especially taking Thomas Gleixner's input (http://thread.gmane.org/gmane.linux.kernel/1703405/focus=2952) into account. I also got some further offlist input from Darren Hart, Torvald Riegel, and Davidlohr Bueso that has been incorporated into the revised draft. Other than that, I got some useful info out of Ulrich Drepper's paper (cited at the end of the page) and one or two web pages (cited in the page source).
The page has now increased in size by a factor of about 5, but is far from complete. In particular, as I reworked the page, there were many details that I was not 100% certain of, and I have added FIXME markers to the page source. In addition, Torvald added some text, and a few more FIXMEs. Some of the FIXMEs are trivial, as in: I'd like confirmation that I have correctly captured a technical detail. Others are more substantial, probably requiring the addition of further text. I appreciate that there are probably other things that can be improved in the page. (Torvald and Darren have some ideas.) However, before growing the page any further, I would like to resolve as many of the FIXMEs (and any other problems that people see) as possible in the existing text. I need help with that. (And I know that dealing with that help, if I get it, will in itself will be quite a task to deal with, which is why I have been delaying it for many weeks now, as my time has been rather limited recently.) So, please take a look at the page below. At this point, I would most especially appreciate help with the FIXMEs. Cheers, Michael ===== .\" Page by b.hubert .\" and Copyright (C) 2015, Thomas Gleixner <t...@linutronix.de> .\" and Copyright (C) 2015, Michael Kerrisk <mtk.manpa...@gmail.com> .\" .\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE) .\" may be freely modified and distributed .\" %%%LICENSE_END .\" .\" Niki A. Rahimi (LTC Security Development, narah...@us.ibm.com) .\" added ERRORS section. .\" .\" Modified 2004-06-17 mtk .\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE .\" .\" FIXME Still to integrate are some points from Torvald Riegel's mail of .\" 2015-01-23: .\" http://thread.gmane.org/gmane.linux.kernel/1703405/focus=7977 .\" .\" FIXME Do we need add some text regarding Torvald Riegel's 2015-01-24 mail .\" at http://thread.gmane.org/gmane.linux.kernel/1703405/focus=1873242 .\" .TH FUTEX 2 2014-05-21 "Linux" "Linux Programmer's Manual" .SH NAME futex \- fast user-space locking .SH SYNOPSIS .nf .sp .B "#include <linux/futex.h>" .B "#include <sys/time.h>" .sp .BI "int futex(int *" uaddr ", int " futex_op ", int " val , .BI " const struct timespec *" timeout , \ " \fR /* or: \fBu32 \fIval2\fP */ .BI " int *" uaddr2 ", int " val3 ); .fi .IR Note : There is no glibc wrapper for this system call; see NOTES. .SH DESCRIPTION .PP The .BR 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 particular condition. The condition is represented by the futex word, which is an address in memory supplied to the .BR futex () system call, and the value at this memory location. (While the virtual addresses for the same memory in separate processes may not be equal, the kernel maps them internally so that the same memory mapped in different locations will correspond for .BR 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 operation that potentially changes shared memory, blocking via a futex is an atomic compare-and-block operation. See NOTES for a detailed specification 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 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. 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 .BR 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. 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 .BR FUTEX_WAIT , described below, are being performed on a particular futex word. .\" .SS Arguments The .I 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 .I futex_op argument; .IR val is a value whose meaning and purpose depends on .IR futex_op . The remaining arguments .RI ( timeout , .IR uaddr2 , and .IR 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 .I timeout argument is a pointer to a .IR timespec structure that specifies a timeout for the operation. However, notwithstanding the prototype shown above, for some operations, this argument is instead a four-byte integer whose meaning is determined by the operation. For these operations, the kernel casts the .I timeout value to .IR u32 , and in the remainder of this page, this argument is referred to as .I val2 when interpreted in this fashion. Where it is required, the .IR uaddr2 argument is a pointer to a second futex word that is employed by the operation. The interpretation of the final integer argument, .IR val3 , depends on the operation. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SS Futex operations The .I 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 .I futex_op are as follows: .TP .BR FUTEX_PRIVATE_FLAG " (since Linux 2.6.22)" .\" commit 34f01cc1f512fa783302982776895c73714ebbc2 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 reference counts on file backing store, and so on. As a convenience, .IR <linux/futex.h> defines a set of constants with the suffix .BR _PRIVATE that are equivalents of all of the operations listed below, .\" except the obsolete FUTEX_FD, for which the "private" flag was .\" meaningless but with the .BR FUTEX_PRIVATE_FLAG ORed into the constant value. Thus, there are .BR FUTEX_WAIT_PRIVATE , .BR FUTEX_WAKE_PRIVATE , and so on. .TP .BR FUTEX_CLOCK_REALTIME " (since Linux 2.6.28)" .\" commit 1acdac104668a0834cfa267de9946fac7764d486 This option bit can be employed only with the .BR FUTEX_WAIT_BITSET and .BR FUTEX_WAIT_REQUEUE_PI operations. If this option is set, the kernel treats .I timeout as an absolute time based on .BR CLOCK_REALTIME . If this option is not set, the kernel treats .I timeout as relative time, .\" FIXME XXX I added CLOCK_MONOTONIC here. Okay? measured against the .BR CLOCK_MONOTONIC clock. .PP The operation specified in .I futex_op is one of the following: .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAIT " (since Linux 2.6.0)" .\" Strictly speaking, since some time in 2.5.x This operation tests that the value at the futex word pointed to by the address .I uaddr still contains the expected value .IR val , and if so, then sleeps awaiting .B 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 considered a waiter on this futex word. If the futex value does not match .IR val , then the call fails immediately with the error .BR 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 .BR FUTEX_WAKE operation (or similar wake-up) after the value change and before this .BR FUTEX_WAIT operation, then the latter will observe the value change and will not start to sleep. If the .I timeout argument is non-NULL, its contents specify a relative timeout for the wait, .\" FIXME XXX I added CLOCK_MONOTONIC here. Okay? measured according to the .BR 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 .I timeout is NULL, the call blocks indefinitely. The arguments .I uaddr2 and .I val3 are ignored. .\" FIXME(Torvald) I think we should remove this. Or maybe adapt to a .\" different example. .\" For .\" .BR futex (7), .\" this call is executed if decrementing the count gave a negative value .\" (indicating contention), .\" and will sleep until another process or thread releases .\" the futex and executes the .\" .B FUTEX_WAKE .\" operation. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAKE " (since Linux 2.6.0)" .\" Strictly speaking, since Linux 2.5.x This operation wakes at most .I val .\" FIXME(Torvald) I believe FUTEX_WAIT_BITSET waiters, for example, .\" could also be woken (therefore, make it e.g. instead of i.e.)? of the waiters that are waiting (e.g., inside .BR FUTEX_WAIT ) on the futex word at the address .IR uaddr . Most commonly, .I val is specified as either 1 (wake up a single waiter) or .BR INT_MAX (wake up all waiters). .\" FIXME Please confirm that the following is correct: 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 .IR timeout , .IR uaddr2 , and .I val3 are ignored. .\" FIXME(Torvald) I think we should remove this. Or maybe adapt to .\" a different example. .\" For .\" .BR futex (7), .\" this is executed if incrementing the count showed that .\" there were waiters, .\" once the futex value has been set to 1 .\" (indicating that it is available). .\" .\" FIXME How does "incrementing the count show that there were waiters"? .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_FD " (from Linux 2.6.0 up to and including Linux 2.6.25)" .\" Strictly speaking, from Linux 2.5.x to 2.6.25 This operation creates a file descriptor that is associated with the futex at .IR uaddr . The caller must close the returned file descriptor after use. When another process or thread performs a .BR FUTEX_WAKE on the futex word, the file descriptor indicates as being readable with .BR select (2), .BR poll (2), and .BR epoll (7) The file descriptor can be used to obtain asynchronous notifications: if .I val is nonzero, then when another process or thread executes a .BR FUTEX_WAKE , the caller will receive the signal number that was passed in .IR val . The arguments .IR timeout , .I uaddr2 and .I val3 are ignored. .\" FIXME(Torvald) We never define "upped". Maybe just remove the .\" following sentence? To prevent race conditions, the caller should test if the futex has been upped after .B FUTEX_FD returns. Because it was inherently racy, .B FUTEX_FD has been removed .\" commit 82af7aca56c67061420d618cc5a30f0fd4106b80 from Linux 2.6.26 onward. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_REQUEUE " (since Linux 2.6.0)" .\" Strictly speaking: from Linux 2.5.70 .\" FIXME(Torvald) Is there some indication that it is broken in general, .\" or is this comment implicitly speaking about the condvar (?) use case? .\" If the latter we might want to weaken the advice a little. .IR "Avoid using this operation" . It is broken for its intended purpose. Use .BR FUTEX_CMP_REQUEUE instead. This operation performs the same task as .BR FUTEX_CMP_REQUEUE , except that no check is made using the value in .IR val3 . (The argument .I val3 is ignored.) .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)" This operation first checks whether the location .I uaddr still contains the value .IR val3 . If not, the operation fails with the error .BR EAGAIN . Otherwise, the operation wakes up a maximum of .I val waiters that are waiting on the futex at .IR uaddr . If there are more than .I val waiters, then the remaining waiters are removed from the wait queue of the source futex at .I uaddr and added to the wait queue of the target futex at .IR uaddr2 . The .I val2 argument specifies an upper limit on the number of waiters that are requeued to the futex at .IR uaddr2 . .\" FIXME(Torvald) Is this correct? Or is just the decision which .\" threads to wake or requeue part of the atomic operation? The load from .I uaddr is an atomic memory access (i.e., using atomic machine instructions of the respective architecture). This load, the comparison with .IR 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 .BR FUTEX_REQUEUE . The difference is that the check of the value at .I 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 .B FUTEX_WAKE is used and all of the waiters that are woken need to acquire another futex. .\" FIXME Please review the following new paragraph to see if it is .\" accurate. Typical values to specify for .I val are 0 or or 1. (Specifying .BR INT_MAX is not useful, because it would make the .BR FUTEX_CMP_REQUEUE operation equivalent to .BR FUTEX_WAKE .) The limit value specified via .I val2 is typically either 1 or .BR INT_MAX . (Specifying the argument as 0 is not useful, because it would make the .BR FUTEX_CMP_REQUEUE operation equivalent to .BR FUTEX_WAIT .) .\" .\" FIXME Here, it would be helpful to have an example of how .\" FUTEX_CMP_REQUEUE might be used, at the same time illustrating .\" why FUTEX_WAKE is unsuitable for the same use case. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .\" FIXME I added a lengthy piece of text on FUTEX_WAKE_OP text, .\" and I'd be happy if someone checked it. .TP .BR FUTEX_WAKE_OP " (since Linux 2.6.14)" .\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721 .\" Author: Jakub Jelinek <ja...@redhat.com> .\" Date: Tue Sep 6 15:16:25 2005 -0700 .\" FIXME(Torvald) The glibc condvar implementation is currently being .\" revised (e.g., to not use an internal lock anymore). .\" It is probably more future-proof to remove this paragraph. 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 .BR 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 associated with the condition variable. .BR FUTEX_WAKE_OP allows such cases to be implemented without leading to high rates of contention and context switching. The .BR FUTEX_WAIT_OP operation is equivalent to execute the following code atomically and totally ordered with respect to other futex operations on any of the two supplied futex words: .in +4n .nf int oldval = *(int *) uaddr2; *(int *) uaddr2 = oldval \fIop\fP \fIoparg\fP; futex(uaddr, FUTEX_WAKE, val, 0, 0, 0); if (oldval \fIcmp\fP \fIcmparg\fP) futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0); .fi .in In other words, .BR FUTEX_WAIT_OP does the following: .RS .IP * 3 saves the original value of the futex word at .IR uaddr2 and performs an operation to modify the value of the futex at .IR uaddr2 ; this is an atomic read-modify-write memory access (i.e., using atomic machine instructions of the respective architecture) .IP * wakes up a maximum of .I val waiters on the futex for the futex word at .IR uaddr ; and .IP * dependent on the results of a test of the original value of the futex word at .IR uaddr2 , wakes up a maximum of .I val2 waiters on the futex for the futex word at .IR uaddr2 . .RE .IP The operation and comparison that are to be performed are encoded in the bits of the argument .IR val3 . Pictorially, the encoding is: .in +8n .nf +---+---+-----------+-----------+ |op |cmp| oparg | cmparg | +---+---+-----------+-----------+ 4 4 12 12 <== # of bits .fi .in Expressed in code, the encoding is: .in +4n .nf #define FUTEX_OP(op, oparg, cmp, cmparg) \\ (((op & 0xf) << 28) | \\ ((cmp & 0xf) << 24) | \\ ((oparg & 0xfff) << 12) | \\ (cmparg & 0xfff)) .fi .in In the above, .I op and .I cmp are each one of the codes listed below. The .I oparg and .I cmparg components are literal numeric values, except as noted below. The .I op component has one of the following values: .in +4n .nf 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; */ .fi .in In addition, bit-wise ORing the following value into .I op causes .IR "(1\ <<\ oparg)" to be used as the operand: .in +4n .nf FUTEX_OP_ARG_SHIFT 8 /* Use (1 << oparg) as operand */ .fi .in The .I cmp field is one of the following: .in +4n .nf 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 */ .fi .in The return value of .BR FUTEX_WAKE_OP is the sum of the number of waiters woken on the futex .IR uaddr plus the number of waiters woken on the futex .IR uaddr2 . .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAIT_BITSET " (since Linux 2.6.25)" .\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d This operation is like .BR FUTEX_WAIT except that .I 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 .BR FUTEX_WAKE_BITSET for further details. The .BR FUTEX_WAIT_BITSET operation also interprets the .I timeout argument differently from .BR FUTEX_WAIT . See the discussion of .BR FUTEX_CLOCK_REALTIME , above. The .I uaddr2 argument is ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAKE_BITSET " (since Linux 2.6.25)" .\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d This operation is the same as .BR FUTEX_WAKE except that the .I 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 .IR val3 ) and the bitset which is stored in the kernel-internal state of the waiter (the "wait" bitset that is set using .BR 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. .\" FIXME XXX Is this paragraph that I added okay? The effect of .BR FUTEX_WAIT_BITSET and .BR 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 multiplexing 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). .\" According to http://locklessinc.com/articles/futex_cheat_sheet/: .\" .\" "The original reason for the addition of these extensions .\" was to improve the performance of pthread read-write locks .\" in glibc. However, the pthreads library no longer uses the .\" same locking algorithm, and these extensions are not used .\" without the bitset parameter being all ones. .\" .\" The page goes on to note that the FUTEX_WAIT_BITSET operation .\" is nevertheless used (with a bitset of all ones) in order to .\" obtain the absolute timeout functionality that is useful .\" for efficiently implementing Pthreads APIs (which use absolute .\" timeouts); FUTEX_WAIT provides only relative timeouts. The .I uaddr2 and .I timeout arguments are ignored. The .BR FUTEX_WAIT and .BR FUTEX_WAKE operations correspond to .BR FUTEX_WAIT_BITSET and .BR FUTEX_WAKE_BITSET operations where the bitsets are all ones. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SS 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-priority 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. .\" FIXME XXX The following is my attempt at a definition of PI futexes, .\" based on mail discussions with Darren Hart. Does it seem okay? >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, .BR FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , and .BR FUTEX_CMP_REQUEUE_PI ). .\" Quoting Darren Hart: .\" These opcodes paired with the PI futex value policy (described below) .\" defines a "futex" as PI aware. These were created very specifically .\" in support of PI pthread_mutexes, so it makes a lot more sense to .\" talk about a PI aware pthread_mutex, than a PI aware futex, since .\" there is a lot of policy and scaffolding that has to be built up .\" around it to use it properly (this is what a PI pthread_mutex is). .\" FIXME XXX ===== Start of adapted Hart/Guniguntala text ===== .\" The following text is drawn from the Hart/Guniguntala paper .\" (listed in SEE ALSO), but I have reworded some pieces .\" significantly. Please check it. .\" 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: .IP * 3 If the lock is not acquired, the futex word's value shall be 0. .IP * If the lock is acquired, the futex word's value shall be the thread ID (TID; see .BR gettid (2)) of the owning thread. .IP * .\" FIXME XXX In the following line, I added "the lock is owned and". Okay? If the lock is owned and there are threads contending for the lock, then the .B FUTEX_WAITERS bit shall be set in the futex word's value; in other words, this value is: FUTEX_WAITERS | TID .PP Note that a PI futex word never just has the value .BR 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 .I 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 .B FUTEX_LOCK_PI operation to acquire the lock. If other threads are waiting for the lock, then the .B FUTEX_WAITERS bit is set in the futex value; in this case, the lock owner must employ the .B FUTEX_UNLOCK_PI operation to release the lock. In the cases where callers are forced into the kernel (i.e., required to perform a .BR 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. .\" FIXME ===== End of adapted Hart/Guniguntala text ===== It is important to note .\" FIXME We need some explanation here of *why* it is important to .\" note this. Can someone explain? that the kernel will update the futex word's value prior to returning to user space. Unlike the other futex operations described above, the PI futex operations are designed for the implementation of very specific IPC mechanisms. .\" .\" FIXME XXX In discussing errors for FUTEX_CMP_REQUEUE_PI, Darren Hart .\" made the observation that "EINVAL is returned if the non-pi .\" to pi or op pairing semantics are violated." .\" Probably there needs to be a general statement about this .\" requirement, probably located at about this point in the page. .\" Darren, care to take a shot at this? .\" .\" FIXME Somewhere on this page (I guess under the discussion of PI .\" futexes) we need a discussion of the FUTEX_OWNER_DIED bit. .\" Can someone propose a text? PI futexes are operated on by specifying one of the following values in .IR futex_op : .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_LOCK_PI " (since Linux 2.6.18)" .\" commit c87e2837be82df479a6bae9f155c43516d2feebc .\" .\" FIXME I did some significant rewording of tglx's text. .\" Please check, in case I injected errors. .\" 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\(emspecifically, because it contained the namespace-specific TID of the lock owner. .\" FIXME In the preceding line, what does "namespace-specific" mean? .\" (I kept those words from tglx.) .\" That is, what kind of namespace are we talking about? .\" (I suppose we are talking PID namespaces here, but I want to .\" be sure.) The operation checks the value of the futex word at the address .IR uaddr . If the value is 0, then the kernel tries to atomically set the futex value to the caller's TID. If that fails, .\" FIXME What would be the cause of failure? or the futex word's value is nonzero, the kernel atomically sets the .B 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, .\" FIXME Could I get a bit more detail on the next two lines? .\" What is "creates or reuses kernel state" about? creates or reuses kernel state on behalf of the owner and attaches the waiter to it. .\" FIXME In the next line, what type of "priority" are we talking about? .\" Realtime priorities for SCHED_FIFO and SCHED_RR? .\" Or something else? The enqueueing of the waiter is in descending priority order if more than one waiter exists. .\" FIXME What does "bandwidth" refer to in the next line? The owner inherits either the priority or the bandwidth of the waiter. .\" FIXME In the preceding line, what determines whether the .\" owner inherits the priority versus the bandwidth? .\" .\" FIXME Could I get some help translating the next sentence into .\" something that user-space developers (and I) can understand? .\" In particular, what are "nested locks" in this context? This inheritance follows the lock chain in the case of nested locking and performs deadlock detection. .\" FIXME tglx says "The timeout argument is handled as described in .\" FUTEX_WAIT." However, it appears to me that this is not right. .\" Is the following formulation correct? The .I timeout argument provides a timeout for the lock attempt. It is interpreted as an absolute time, measured against the .BR CLOCK_REALTIME clock. If .I timeout is NULL, the operation will block indefinitely. The .IR uaddr2 , .IR val , and .IR val3 arguments are ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_TRYLOCK_PI " (since Linux 2.6.18)" .\" commit c87e2837be82df479a6bae9f155c43516d2feebc This operation tries to acquire the futex at .IR uaddr . .\" 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? .\" .\" FIXME(Torvald) Additionally, we claim above that just FUTEX_WAITERS .\" is never an allowed state. It deals with the situation where the TID value at .I uaddr is 0, but the .B FUTEX_WAITERS bit is set. .\" FIXME How does the situation in the previous sentence come about? .\" Probably it would be helpful to say something about that in .\" the man page. .\" FIXME And *how* does FUTEX_TRYLOCK_PI deal with this situation? User space cannot handle this condition in a race-free manner The .IR uaddr2 , .IR val , .IR timeout , and .IR val3 arguments are ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_UNLOCK_PI " (since Linux 2.6.18)" .\" commit c87e2837be82df479a6bae9f155c43516d2feebc This operation wakes the top priority waiter that is waiting in .B FUTEX_LOCK_PI on the futex address provided by the .I uaddr argument. This is called when the user space value at .I uaddr cannot be changed atomically from a TID (of the owner) to 0. The .IR uaddr2 , .IR val , .IR timeout , and .IR val3 arguments are ignored. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_CMP_REQUEUE_PI " (since Linux 2.6.31)" .\" commit 52400ba946759af28442dee6265c5c0180ac7122 This operation is a PI-aware variant of .BR FUTEX_CMP_REQUEUE . It requeues waiters that are blocked via .B FUTEX_WAIT_REQUEUE_PI on .I uaddr from a non-PI source futex .RI ( uaddr ) to a PI target futex .RI ( uaddr2 ). As with .BR FUTEX_CMP_REQUEUE , this operation wakes up a maximum of .I val waiters that are waiting on the futex at .IR uaddr . However, for .BR FUTEX_CMP_REQUEUE_PI , .I 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 .I uaddr and added to the wait queue of the target futex at .IR uaddr2 . The .I val2 .\" val2 is the cap on the number of requeued waiters. .\" In the glibc pthread_cond_broadcast() implementation, this argument .\" is specified as INT_MAX, and for pthread_cond_signal() it is 0. and .I val3 arguments serve the same purposes as for .BR FUTEX_CMP_REQUEUE . .\" .\" FIXME The page at http://locklessinc.com/articles/futex_cheat_sheet/ .\" notes that "priority-inheritance Futex to priority-inheritance .\" Futex requeues are currently unsupported". Do we need to say .\" something in the man page about that? .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP .BR FUTEX_WAIT_REQUEUE_PI " (since Linux 2.6.31)" .\" commit 52400ba946759af28442dee6265c5c0180ac7122 .\" .\" FIXME I find the next sentence (from tglx) pretty hard to grok. .\" Could someone explain it a bit more? Wait operation to wait on a non-PI futex at .I uaddr and potentially be requeued onto a PI futex at .IR uaddr2 . The wait operation on .I uaddr is the same as .BR FUTEX_WAIT . .\" .\" FIXME I'm not quite clear on the meaning of the following sentence. .\" Is this trying to say that while blocked in a .\" FUTEX_WAIT_REQUEUE_PI, it could happen that another .\" task does a FUTEX_WAKE on uaddr that simply causes .\" a normal wake, with the result that the FUTEX_WAIT_REQUEUE_PI .\" does not complete? What happens then to the FUTEX_WAIT_REQUEUE_PI .\" opertion? Does it remain blocked, or does it unblock .\" In which case, what does user space see? The waiter can be removed from the wait on .I uaddr via .BR FUTEX_WAKE without requeueing on .IR uaddr2 . .\" FIXME Please check the following. tglx said "The timeout argument .\" is handled as described in FUTEX_WAIT.", but the truth is .\" as below, AFAICS If .I timeout is not NULL, it specifies a timeout for the wait operation; this timeout is interpreted as outlined above in the description of the .BR FUTEX_CLOCK_REALTIME option. If .I timeout is NULL, the operation can block indefinitely. The .I val3 argument is ignored. .\" FIXME Re the preceding sentence... Actually 'val3' is internally set to .\" FUTEX_BITSET_MATCH_ANY before calling futex_wait_requeue_pi(). .\" I'm not sure we need to say anything about this though. .\" Comments? The .BR FUTEX_WAIT_REQUEUE_PI and .BR FUTEX_CMP_REQUEUE_PI were added to support a fairly specific use case: support for priority-inheritance-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 .BR FUTEX_WAIT_REQUEUE_PI operation, the user-space application pre-specifies the target of the requeue that takes place in the .BR FUTEX_CMP_REQUEUE_PI operation. .\" .\" Darren Hart notes that a patch to allow glibc to fully support .\" PI-aware pthreads condition variables has not yet been accepted into .\" glibc. The story is complex, and can be found at .\" https://sourceware.org/bugzilla/show_bug.cgi?id=11588 .\" Darren notes that in the meantime, the patch is shipped with various .\" PREEMPT_RT-enabled Linux systems. .\" .\" Related to the preceding, Darren proposed that somewhere, man-pages .\" should document the following point: .\" .\" While the Linux kernel, since 2.6.31, supports requeueing of .\" priority-inheritance (PI) aware mutexes via the .\" FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI futex operations, .\" the glibc implementation does not yet take full advantage of this. .\" Specifically, the condvar internal data lock remains a non-PI aware .\" mutex, regardless of the type of the pthread_mutex associated with .\" the condvar. This can lead to an unbounded priority inversion on .\" the internal data lock even when associating a PI aware .\" pthread_mutex with a condvar during a pthread_cond*_wait .\" operation. For this reason, it is not recommended to rely on .\" priority inheritance when using pthread condition variables. .\" .\" The problem is that the obvious location for this text is .\" the pthread_cond*wait(3) man page. However, such a man page .\" does not currently exist. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH RETURN VALUE .PP In the event of an error, all operations return \-1 and set .I errno to indicate the cause of the error. The return value on success depends on the operation, as described in the following list: .TP .B 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, callers 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. .TP .B FUTEX_WAKE Returns the number of waiters that were woken up. .TP .B FUTEX_FD Returns the new file descriptor associated with the futex. .TP .B FUTEX_REQUEUE Returns the number of waiters that were woken up. .TP .B FUTEX_CMP_REQUEUE Returns the total number of waiters that were woken up or requeued to the futex for the futex word at .IR uaddr2 . If this value is greater than .IR val , then difference is the number of waiters requeued to the futex for the futex word at .IR uaddr2 . .TP .B 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 .I uaddr and .IR uaddr2 . .TP .B FUTEX_WAIT_BITSET Returns 0 if the caller was woken up. See .B FUTEX_WAIT for how to interpret this correctly in practice. .TP .B FUTEX_WAKE_BITSET Returns the number of waiters that were woken up. .TP .B FUTEX_LOCK_PI Returns 0 if the futex was successfully locked. .TP .B FUTEX_TRYLOCK_PI Returns 0 if the futex was successfully locked. .TP .B FUTEX_UNLOCK_PI Returns 0 if the futex was successfully unlocked. .TP .B FUTEX_CMP_REQUEUE_PI Returns the total number of waiters that were woken up or requeued to the futex for the futex word at .IR uaddr2 . If this value is greater than .IR val , then difference is the number of waiters requeued to the futex for the futex word at .IR uaddr2 . .TP .B FUTEX_WAIT_REQUEUE_PI Returns 0 if the caller was successfully requeued to the futex for the futex word at .IR uaddr2 . .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH ERRORS .TP .B EACCES No read access to the memory of a futex word. .TP .B EAGAIN .RB ( FUTEX_WAIT , .BR FUTEX_WAIT_BITSET , .BR FUTEX_WAIT_REQUEUE_PI ) The value pointed to by .I uaddr was not equal to the expected value .I val at the time of the call. .BR Note : on Linux, the symbolic names .B EAGAIN and .B EWOULDBLOCK (both of which appear in different parts of the kernel futex code) have the same value. .TP .B EAGAIN .RB ( FUTEX_CMP_REQUEUE , .BR FUTEX_CMP_REQUEUE_PI ) The value pointed to by .I uaddr is not equal to the expected value .IR val3 . .\" FIXME: Is the following sentence correct? .\" I would prefer to remove this sentence. --trie...@redhat.com (This probably indicates a race; use the safe .B FUTEX_WAKE now.) .\" .\" FIXME XXX Should there be an EAGAIN case for FUTEX_TRYLOCK_PI? .\" It seems so, looking at the handling of the rt_mutex_trylock() .\" call in futex_lock_pi() .\" (Davidlohr also thinks so.) .\" .TP .BR EAGAIN .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The futex owner thread ID of .I uaddr (for .BR FUTEX_CMP_REQUEUE_PI : .IR uaddr2 ) is about to exit, but has not yet handled the internal state cleanup. Try again. .TP .BR EDEADLK .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The futex word at .I uaddr is already locked by the caller. .TP .BR EDEADLK .\" FIXME I reworded tglx's text somewhat; is the following okay? .\" FIXME XXX I see that kernel/locking/rtmutex.c uses EDEADLK in some .\" iplaces, and EDEADLOCK in others. On almost all architectures .\" these constants are synonymous. Is there a reason that both .\" names are used? .RB ( FUTEX_CMP_REQUEUE_PI ) While requeueing a waiter to the PI futex for the futex word at .IR uaddr2 , the kernel detected a deadlock. .TP .B EFAULT A required pointer argument (i.e., .IR uaddr , .IR uaddr2 , or .IR timeout ) did not point to a valid user-space address. .TP .B EINTR A .B FUTEX_WAIT or .B FUTEX_WAIT_BITSET operation was interrupted by a signal (see .BR 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. .TP .B EINVAL The operation in .IR futex_op is one of those that employs a timeout, but the supplied .I timeout argument was invalid .RI ( tv_sec was less than zero, or .IR tv_nsec was not less than 1000,000,000). .TP .B EINVAL The operation specified in .IR futex_op employs one or both of the pointers .I uaddr and .IR uaddr2 , but one of these does not point to a valid object\(emthat is, the address is not four-byte-aligned. .TP .B EINVAL .RB ( FUTEX_WAIT_BITSET , .BR FUTEX_WAKE_BITSET ) The bitset supplied in .IR val3 is zero. .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) .I uaddr equals .IR uaddr2 (i.e., an attempt was made to requeue to the same futex). .TP .BR EINVAL .RB ( FUTEX_FD ) The signal number supplied in .I val is invalid. .TP .B EINVAL .RB ( FUTEX_WAKE , .BR FUTEX_WAKE_OP , .BR FUTEX_WAKE_BITSET , .BR FUTEX_REQUEUE , .BR FUTEX_CMP_REQUEUE ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state\(emthat is, it detected a waiter which waits in .BR FUTEX_LOCK_PI on .IR uaddr . .TP .B EINVAL .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_UNLOCK_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state. This indicates either state corruption .\" FIXME tglx did not mention the "state corruption" for FUTEX_UNLOCK_PI. .\" Does that case also apply for FUTEX_UNLOCK_PI? or that the kernel found a waiter on .I uaddr which is waiting via .BR FUTEX_WAIT or .BR FUTEX_WAIT_BITSET . .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr2 and the kernel state; that is, the kernel detected a waiter which waits via .BR FUTEX_WAIT .\" FIXME tglx did not mention FUTEX_WAIT_BITSET here, .\" but should that not also be included here? on .IR uaddr2 . .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state; that is, the kernel detected a waiter which waits via .BR FUTEX_WAIT or .BR FUTEX_WAIT_BITESET on .IR uaddr . .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state; that is, the kernel detected a waiter which waits on .I uaddr via .BR FUTEX_LOCK_PI (instead of .BR FUTEX_WAIT_REQUEUE_PI ). .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) .\" FIXME XXX The following is a reworded version of Darren Hart's text. .\" Please check that I did not introduce any errors. An attempt was made to requeue a waiter to a futex other than that specified by the matching .B FUTEX_WAIT_REQUEUE_PI call for that waiter. .TP .B EINVAL .RB ( FUTEX_CMP_REQUEUE_PI ) The .I val argument is not 1. .TP .B EINVAL Invalid argument. .TP .BR ENOMEM .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The kernel could not allocate memory to hold state information. .TP .B ENFILE .RB ( FUTEX_FD ) The system limit on the total number of open files has been reached. .TP .B ENOSYS Invalid operation specified in .IR futex_op . .TP .B ENOSYS The .BR FUTEX_CLOCK_REALTIME option was specified in .IR futex_op , but the accompanying operation was neither .BR FUTEX_WAIT_BITSET nor .BR FUTEX_WAIT_REQUEUE_PI . .TP .BR ENOSYS .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_UNLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI , .BR 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. .TP .BR EPERM .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The caller is not allowed to attach itself to the futex at .I uaddr (for .BR FUTEX_CMP_REQUEUE_PI : the futex at .IR uaddr2 ). (This may be caused by a state corruption in user space.) .TP .BR EPERM .RB ( FUTEX_UNLOCK_PI ) The caller does not own the lock represented by the futex word. .TP .BR ESRCH .RB ( FUTEX_LOCK_PI , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) .\" FIXME I reworded the following sentence a bit differently from .\" tglx's formulation. Is it okay? The thread ID in the futex word at .I uaddr does not exist. .TP .BR ESRCH .RB ( FUTEX_CMP_REQUEUE_PI ) .\" FIXME I reworded the following sentence a bit differently from .\" tglx's formulation. Is it okay? The thread ID in the futex word at .I uaddr2 does not exist. .TP .B ETIMEDOUT The operation in .IR futex_op employed the timeout specified in .IR timeout , and the timeout expired before the operation completed. .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH VERSIONS .PP 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\(emmessy, especially on the s390 architecture. .SH CONFORMING TO This system call is Linux-specific. .SH NOTES Glibc does not provide a wrapper for this system call; call it using .BR syscall (2). .\" TODO FIXME(Torvald) Above, we cite this section and claim it contains .\" details on the synchronization semantics; add the C11 equivalents .\" here (or whatever we find consensus for). .\" .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .SH EXAMPLE .\" FIXME Is it worth having an example program? .\" FIXME Anything obviously broken in the example program? .\" 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 .IR 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: .in +4n .nf $ \fB./futex_demo\fP 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 .fi .in .SS Program source \& .nf /* 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 \(aqnum\-loops\(aq 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 \(aqfutexp\(aq: 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 \(aqfutexp\(aq: 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); } .fi .SH SEE ALSO .ad l .BR get_robust_list (2), .BR restart_syscall (2), .BR futex (7) .PP The following kernel source files: .IP * 2 .I Documentation/pi-futex.txt .IP * .I Documentation/futex-requeue-pi.txt .IP * .I Documentation/locking/rt-mutex.txt .IP * .I Documentation/locking/rt-mutex-design.txt .IP * .I Documentation/robust-futex-ABI.txt .PP Franke, H., Russell, R., and Kirwood, M., 2002. \fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP (from proceedings of the Ottawa Linux Symposium 2002), .br .UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf .UE Hart, D., 2009. \fIA futex overview and update\fP, .UR http://lwn.net/Articles/360699/ .UE Hart, D. and Guniguntala, D., 2009. \fIRequeue-PI: Making Glibc Condvars PI-Aware\fP (from proceedings of the 2009 Real-Time Linux Workshop), .UR http://lwn.net/images/conf/rtlws11/papers/proc/p10.pdf .UE Drepper, U., 2011. \fIFutexes Are Tricky\fP, .UR http://www.akkadia.org/drepper/futex.pdf .UE .PP Futex example library, futex-*.tar.bz2 at .br .UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/ .UE .\" .\" FIXME Are there any other resources that should be listed .\" in the SEE ALSO section? .\" FIXME(Torvald) We should probably refer to the glibc code here, in .\" particular the glibc-internal futex wrapper functions that are .\" WIP, and the generic pthread_mutex_t and perhaps condvar .\" implementations. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
