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It is similar to the FUTEX_WAKE operation, with addr1 referring to the wait-queue to transfer sleepers from. For 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). ENOSYS The FUTEX_CLOCK_REALTIME option was specified in futex_op, but the accompanying operation was neither FUTEX_WAIT, FUTEX_WAIT_BITSET, nor FUTEX_WAIT_REQUEUE_PI. This causes a small inefficiency where the wake-up code may call into the kernel more than strictly necessary, but this is much better than having missed wake-ups and dead-lock.

FUTEX_WAIT_BITSET Returns 0 if the caller was woken up. Linux Cross Reference Free Electrons Embedded Linux Experts •source navigation •diff markup •identifier search •freetext search • Version: 2.0.402. Linux/kernel/futex.c 1 /* 2 * Fast Userspace Mutexes (which I call "Futexes!"). This means that missed wakeups are unavoidable. That way only the waiters that you are interested in need to be scanned for wakeups.

ret : 1; 1219 } 1220 1221 /* 1222 * First waiter. For anonymous pages, it 557 * does not matter if the page splits in the future as the key is 558 * based on the address. The fourth is usually a timeout, but in some cases may be a second integer argument. (This is an example where the slow accretion of features to the Futex API is This indicates either state corruption or that the kernel found a waiter on uaddr which is waiting via FUTEX_WAIT or FUTEX_WAIT_BITSET.

This would add a few extra error cases where one but not the other of the two pages is invalid. For private mappings, it's (uaddr, current->mm). 484 * We can usually work out the index without swapping in the page. 485 * 486 * lock_page() might sleep, the caller should not After the RT-mutex is acquired, the futex value is updated accordingly, before the calling thread returns to user space. Are you sure you posted the correct source?

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. Linux 2016-07-17 FUTEX(2) Copyright and license for this manual page HTML rendering created 2016-10-08 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. If no matching 942 * thread is found then it indicates that the owner TID has died. 943 * 944 * [3] Invalid. Other futex() operations can be used to wake any processes or threads waiting for a particular condition.

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- Try again. I simply love this utility.🙂 Strace is the tool that traces the execution of system calls. Previously 659 * this reference was taken by ihold under the page lock 660 * pinning the inode in place so i_lock was unnecessary.

Conversely, to discover its location, the sys_get_robust_list() may be used. This could have been the first step to introduce a uniform way to wait on different objects at the same time like it is possible on Windows with WaitForMultipleObjects. October 11, 2013 Building & Running the sample projects ofAndroid July 30, 2013 Adding Android Support in Eclipse onLinux July 19, 2013 Categories Android (1) General (3) How To (4) Linux Callers 1260 * must ensure to later call wake_up_q() for the actual 1261 * wakeups to occur. 1262 */ 1263 static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q) 1264 { 1265

A call to unqueue_me() must 2039 * be paired with exactly one earlier call to queue_me(). 2040 * 2041 * Return: 2042 * 1 - if the futex_q was still queued Most programmers should use the pthreads interface. Each key may have multiple futex_q structures, one for each task 258 * waiting on a futex. 259 */ 260 struct futex_hash_bucket { 261 atomic_t waiters; 262 spinlock_t lock; 263 struct The fact that the kernel doesn't need to be entered increases efficiency.

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. This allows the kernel to make some additional performance optimizations. FUTEX_CMP_REQUEUE (since Linux 2.6.7) There was a race in the intended use of FUTEX_REQUEUE, so FUTEX_CMP_REQUEUE was introduced. HdcDst - look at the man page for eventfd.

Note how the above code fragment unconditionally sets the mutex into the used (1-bit set), and contended (256-bit set). A 1279 * memory barrier is required here to prevent the following 1280 * store to lock_ptr from getting ahead of the plist_del. 1281 */ 1282 smp_wmb(); 1283 q->lock_ptr = NULL; 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. EINVAL (FUTEX_CMP_REQUEUE_PI) uaddr equals uaddr2 (i.e., an attempt was made to requeue to the same futex).

The operation to execute is passed via the op argument, along with a value val. Privacy policy About Wikipedia Disclaimers Contact Wikipedia Developers Cookie statement Mobile view futex(2) - Linux man page Name futex - fast user-space locking Synopsis #include #include int futex(int *uaddr, If timeout is not NULL, the structure it points to specifies an absolute timeout for the wait operation. If a timeout is required, then the timeout argument can be used signifying an absolute timeout, otherwise a NULL value signifies that the thread should wait forever.

It is invoked when a user-space atomic acquire did not succeed because the futex word was not 0. If timeout is NULL, the operation can block indefinitely. 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 We preserve the owner died bit. 1208 */ 1209 newval = uval & FUTEX_OWNER_DIED; 1210 newval |= vpid; 1211 1212 /* The futex requeue_pi code can enforce the waiters bit */

The original reason for the addition of these extensions was to improve the performance of pthread read-write locks in glibc. So in the common case, where the lock is uncontended only userspace atomic instructions are needed. A four-argument system call with the semantics described in this page was introduced in Linux 2.5.40. See also[edit] Synchronization Fetch-and-add Compare and swap References[edit] ^ Torvalds, Linus. "Futex Asynchronous Interface". ^ CVE-2014-3153 ^ "[SECURITY] [DSA 2949-1] linux security update".

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 argument. Priority-inheritance futexes Linux supports priority-inheritance (PI) futexes in order to handle priority-inversion problems that can be encountered with normal futex locks.