*/
int lttng_ust_loaded __attribute__((weak));
+/*
+ * Notes on async-signal-safety of ust lock: a few libc functions are used
+ * which are not strictly async-signal-safe:
+ *
+ * - pthread_setcancelstate
+ * - pthread_mutex_lock
+ * - pthread_mutex_unlock
+ *
+ * As of glibc 2.35, the implementation of pthread_setcancelstate only
+ * touches TLS data, and it appears to be safe to use from signal
+ * handlers. If the libc implementation changes, this will need to be
+ * revisited, and we may ask glibc to provide an async-signal-safe
+ * pthread_setcancelstate.
+ *
+ * As of glibc 2.35, the implementation of pthread_mutex_lock/unlock
+ * for fast mutexes only relies on the pthread_mutex_t structure.
+ * Disabling signals around all uses of this mutex ensures
+ * signal-safety. If the libc implementation changes and eventually uses
+ * other global resources, this will need to be revisited and we may
+ * need to implement our own mutex.
+ */
+
/*
* Return 0 on success, -1 if should quit.
* The lock is taken in both cases.
pid = fork();
URCU_TLS(lttng_ust_nest_count)--;
if (pid > 0) {
- int status;
+ int status, wait_ret;
/*
* Parent: wait for child to return, in which case the
* shared memory map will have been created.
*/
- pid = wait(&status);
- if (pid < 0 || !WIFEXITED(status) || WEXITSTATUS(status) != 0) {
+ wait_ret = waitpid(pid, &status, 0);
+ if (wait_ret < 0 || !WIFEXITED(status) || WEXITSTATUS(status) != 0) {
wait_shm_fd = -1;
goto end;
}
DBG("Waiting for %s apps sessiond", sock_info->name);
/* Wait for futex wakeup */
- if (uatomic_read((int32_t *) sock_info->wait_shm_mmap))
- goto end_wait;
-
- while (lttng_ust_futex_async((int32_t *) sock_info->wait_shm_mmap,
- FUTEX_WAIT, 0, NULL, NULL, 0)) {
+ while (!uatomic_read((int32_t *) sock_info->wait_shm_mmap)) {
+ if (!lttng_ust_futex_async((int32_t *) sock_info->wait_shm_mmap, FUTEX_WAIT, 0, NULL, NULL, 0)) {
+ /*
+ * Prior queued wakeups queued by unrelated code
+ * using the same address can cause futex wait to
+ * return 0 even through the futex value is still
+ * 0 (spurious wakeups). Check the value again
+ * in user-space to validate whether it really
+ * differs from 0.
+ */
+ continue;
+ }
switch (errno) {
- case EWOULDBLOCK:
+ case EAGAIN:
/* Value already changed. */
goto end_wait;
case EINTR:
/* Retry if interrupted by signal. */
- break; /* Get out of switch. */
+ break; /* Get out of switch. Check again. */
case EFAULT:
wait_poll_fallback = 1;
DBG(