mtx_unlock.cpp
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//===-- Linux implementation of the mtx_unlock function -------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "config/linux/syscall.h" // For syscall functions.
#include "include/sys/syscall.h" // For syscall numbers.
#include "include/threads.h" // For mtx_t definition.
#include "src/__support/common.h"
#include "src/threads/linux/thread_utils.h"
#include <linux/futex.h> // For futex operations.
#include <stdatomic.h> // for atomic_compare_exchange_strong.
namespace __llvm_libc {
// The implementation currently handles only plain mutexes.
int LLVM_LIBC_ENTRYPOINT(mtx_unlock)(mtx_t *mutex) {
FutexData *futex_word = reinterpret_cast<FutexData *>(mutex->__internal_data);
while (true) {
uint32_t mutex_status = MS_Waiting;
if (atomic_compare_exchange_strong(futex_word, &mutex_status, MS_Free)) {
// If any thread is waiting to be woken up, then do it.
__llvm_libc::syscall(SYS_futex, futex_word, FUTEX_WAKE_PRIVATE, 1, 0, 0,
0);
return thrd_success;
}
if (mutex_status == MS_Locked) {
// If nobody was waiting at this point, just free it.
if (atomic_compare_exchange_strong(futex_word, &mutex_status, MS_Free))
return thrd_success;
} else {
// This can happen, for example if some thread tries to unlock an already
// free mutex.
return thrd_error;
}
}
}
} // namespace __llvm_libc