![]() WebKit implementation lacks FUTEX_REQUEUE equivalent, although it is possible to add this if needed.You can finally move your WhatsApp chats from Android to iOS Unfortunately, performance in contested case will most likely be slower, as it will use futex(2) underneath anyway to block the calling thread, so there is an additional layer of code and locking that needs to be executed. This makes it possible to store less information in synchronization object and be overall smarter with thread scheduling. Mechanism is basically the same as with futex(2), but since it is implemented in userspace it allows execution of custom user code from inside the queue lock. WebKit's ParkingLot is an example of this. NtWaitForKeyedEvent( keyed_event, &variable->Ptr, FALSE, timeout ) Īnother way is to emulate futex-like functionality in userspace. ![]() Interlocked_xchg_add( (int *)&variable->Ptr, 1 ) RTL_CONDITION_VARIABLE *variable, RTL_SRWLOCK *lock) Int val = interlocked_xchg( (int *)&variable->Ptr, 0 ) ![]() Void WINAPI RtlWakeAllConditionVariable( RTL_CONDITION_VARIABLE *variable ) NtReleaseKeyedEvent( keyed_event, &variable->Ptr, FALSE, NULL) If (interlocked_dec_if_nonzero( (int *)&variable->Ptr )) void WINAPI RtlWakeConditionVariable( RTL_CONDITION_VARIABLE *variable ) Condition variable is just a 4 byte counter of waiters.įollowing code is simplified, you can check the original here. Keyed events are well suited for condition variables, making implemenation is extremely simple. If you want to see more Keyed Events in action, you should check Wine's sources and their implementation of RTL_SRWLOCK and RTL_CONDITION_VARIABLE. Using such API requires having waiter counter in userspace. calling threads is blocked until it actually wakes someone. This mechanism ensures that wake-ups are not missed by making "release" operation blocking e.g. There are two operations: "wait" and "release" (wake). Just as futex(2), keyed events use table of wait queues hashed by address. They provide alternative undocumented API called "Keyed Events". I think OpenBSD partially implements futex(2) too.Įarlier versions of Windows (7, Vista, XP) are special._umtx_op(2) on OS X, FreeBSD (never used those, you can read more here).WaitOnAddress, WakeOnAddress on Windows 8 and later.You can find it here.Īnother decent article that shows usage of futex(2) is here, but this implementation is too "benchmark oriented".Īs for other operating systems, today most provide equivalent of Linux'es futex(2): It explains how futex(2) works and shows how it can be used to implemented common primitives. There is a good document "Futexes Are Tricky" about futexes written by Ulrich Drepper. Languages like Java and C# both use monitors instead of mutexes and condition variables.Īnother futex(2) trick is FUTEX_REQUEUE operation that can be used to move waiters from one futex to another and can be used to make condition varaible very effecient. Such combination of a mutex and a condition variable that only allows "notify" to be called from within critical section is usually called "monitor". If you allow "notify" to be only called from within critical section, additional mutex locking is not required, making implementation simpler and faster. State of condition variable might need to be protected by a separate mutex, so it is common for implementations to keep another mutex within a condition variable. Complexity and performance of implementation depends on how much you want to stick to a "classic" interface, mainly whether you allow "notify" to be called outside of critical section or not. * so it must not be touch - hence the extra CONTESTED state */Ĭondition variables are trickier. * The semaphore has been unlocked and could be deallocated, New = (int) ((unsigned int) val + n + (val ftx, &val, new)) Int new, waiters, val = atomic_load(&sem->ftx) Example below: struct mutex Ītomic_cmpxchg(&sem->ftx, LOCKED, CONTESTED) Using futex(2) it is possible to implement mutex that does not require any syscall if mutex is not contested, but is able to suspend calling thread otherwise. It is very important that FUTEX_WAIT verifies value before sleeping, as this ensures thread does not miss wake-ups. (e.g., inside FUTEX_WAIT) on the futex word at the address uaddr. This operation wakes at most val of the waiters that are waiting Then sleeps waiting for a FUTEX_WAKE operation on the futex word. The address uaddr still contains the expected value val, and if so, This operation tests that the value at the futex word pointed to by Two most commonly used operations are FUTEX_WAIT: I will try to focus on modern implementations.įirst, Linux uses futex syscall ( Fast Userspace mu TEX). I think your question is more about what primitives are used to implement those, and the answer is - it depends.
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