1 //===-- Linux implementation of the mtx_lock function ---------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #include "config/linux/syscall.h" // For syscall functions.
10 #include "include/sys/syscall.h" // For syscall numbers.
11 #include "include/threads.h" // For mtx_t definition.
12 #include "src/__support/common.h"
13 #include "src/threads/linux/thread_utils.h"
14
15 #include <linux/futex.h> // For futex operations.
16 #include <stdatomic.h> // For atomic_compare_exchange_strong.
17
18 namespace __llvm_libc {
19
20 // The implementation currently handles only plain mutexes.
LLVM_LIBC_ENTRYPOINT(mtx_lock)21 int LLVM_LIBC_ENTRYPOINT(mtx_lock)(mtx_t *mutex) {
22 FutexData *futex_data = reinterpret_cast<FutexData *>(mutex->__internal_data);
23 while (true) {
24 uint32_t mutex_status = MS_Free;
25 uint32_t locked_status = MS_Locked;
26
27 if (atomic_compare_exchange_strong(futex_data, &mutex_status, MS_Locked))
28 return thrd_success;
29
30 switch (mutex_status) {
31 case MS_Waiting:
32 // If other threads are waiting already, then join them. Note that the
33 // futex syscall will block if the futex data is still `MS_Waiting` (the
34 // 4th argument to the syscall function below.)
35 __llvm_libc::syscall(SYS_futex, futex_data, FUTEX_WAIT_PRIVATE,
36 MS_Waiting, 0, 0, 0);
37 // Once woken up/unblocked, try everything all over.
38 continue;
39 case MS_Locked:
40 // Mutex has been locked by another thread so set the status to
41 // MS_Waiting.
42 if (atomic_compare_exchange_strong(futex_data, &locked_status,
43 MS_Waiting)) {
44 // If we are able to set the futex data to `MS_Waiting`, then we will
45 // wait for the futex to be woken up. Note again that the following
46 // syscall will block only if the futex data is still `MS_Waiting`.
47 __llvm_libc::syscall(SYS_futex, futex_data, FUTEX_WAIT_PRIVATE,
48 MS_Waiting, 0, 0, 0);
49 }
50 continue;
51 case MS_Free:
52 // If it was MS_Free, we shouldn't be here at all.
53 [[clang::fallthrough]];
54 default:
55 // Mutex status cannot be anything else. So control should not reach
56 // here at all.
57 return thrd_error;
58 }
59 }
60 }
61
62 } // namespace __llvm_libc
63