diff options
| -rw-r--r-- | libc/src/threads/linux/mtx_lock.cpp | 8 | ||||
| -rw-r--r-- | libc/test/src/threads/mtx_test.cpp | 53 |
2 files changed, 60 insertions, 1 deletions
diff --git a/libc/src/threads/linux/mtx_lock.cpp b/libc/src/threads/linux/mtx_lock.cpp index 7ae30a7..39bd329 100644 --- a/libc/src/threads/linux/mtx_lock.cpp +++ b/libc/src/threads/linux/mtx_lock.cpp @@ -21,12 +21,16 @@ namespace __llvm_libc { // The implementation currently handles only plain mutexes. LLVM_LIBC_FUNCTION(int, mtx_lock, (mtx_t * mutex)) { FutexData *futex_data = reinterpret_cast<FutexData *>(mutex->__internal_data); + bool was_waiting = false; while (true) { uint32_t mutex_status = MS_Free; uint32_t locked_status = MS_Locked; - if (atomic_compare_exchange_strong(futex_data, &mutex_status, MS_Locked)) + if (atomic_compare_exchange_strong(futex_data, &mutex_status, MS_Locked)) { + if (was_waiting) + atomic_store(futex_data, MS_Waiting); return thrd_success; + } switch (mutex_status) { case MS_Waiting: @@ -35,6 +39,7 @@ LLVM_LIBC_FUNCTION(int, mtx_lock, (mtx_t * mutex)) { // 4th argument to the syscall function below.) __llvm_libc::syscall(SYS_futex, futex_data, FUTEX_WAIT_PRIVATE, MS_Waiting, 0, 0, 0); + was_waiting = true; // Once woken up/unblocked, try everything all over. continue; case MS_Locked: @@ -47,6 +52,7 @@ LLVM_LIBC_FUNCTION(int, mtx_lock, (mtx_t * mutex)) { // syscall will block only if the futex data is still `MS_Waiting`. __llvm_libc::syscall(SYS_futex, futex_data, FUTEX_WAIT_PRIVATE, MS_Waiting, 0, 0, 0); + was_waiting = true; } continue; case MS_Free: diff --git a/libc/test/src/threads/mtx_test.cpp b/libc/test/src/threads/mtx_test.cpp index 0f3df0e..536ff52 100644 --- a/libc/test/src/threads/mtx_test.cpp +++ b/libc/test/src/threads/mtx_test.cpp @@ -127,3 +127,56 @@ TEST(LlvmLibcMutexTest, WaitAndStep) { __llvm_libc::thrd_join(&thread, &retval); ASSERT_EQ(retval, 0); } + +static constexpr int THREAD_COUNT = 10; +static mtx_t multiple_waiter_lock; +static mtx_t counter_lock; +static int wait_count = 0; + +int waiter_func(void *) { + __llvm_libc::mtx_lock(&counter_lock); + ++wait_count; + __llvm_libc::mtx_unlock(&counter_lock); + + // Block on the waiter lock until the main + // thread unblocks. + __llvm_libc::mtx_lock(&multiple_waiter_lock); + __llvm_libc::mtx_unlock(&multiple_waiter_lock); + + __llvm_libc::mtx_lock(&counter_lock); + --wait_count; + __llvm_libc::mtx_unlock(&counter_lock); + + return 0; +} + +TEST(LlvmLibcMutexTest, MultipleWaiters) { + __llvm_libc::mtx_init(&multiple_waiter_lock, mtx_plain); + __llvm_libc::mtx_init(&counter_lock, mtx_plain); + + __llvm_libc::mtx_lock(&multiple_waiter_lock); + thrd_t waiters[THREAD_COUNT]; + for (int i = 0; i < THREAD_COUNT; ++i) { + __llvm_libc::thrd_create(waiters + i, waiter_func, nullptr); + } + + // Spin until the counter is incremented to the desired + // value. + while (true) { + __llvm_libc::mtx_lock(&counter_lock); + if (wait_count == THREAD_COUNT) { + __llvm_libc::mtx_unlock(&counter_lock); + break; + } + __llvm_libc::mtx_unlock(&counter_lock); + } + + __llvm_libc::mtx_unlock(&multiple_waiter_lock); + + int retval; + for (int i = 0; i < THREAD_COUNT; ++i) { + __llvm_libc::thrd_join(waiters + i, &retval); + } + + ASSERT_EQ(wait_count, 0); +} |