[LEDE-DEV] [PATCH] uloop: use a waker for notifying sigchld and loop cancel events

[Yousong Zhou]

Fix a race condition when do_sigchld, uloop_cancelled were set just
before epoll_wait(timeout=-1), resulting the loop stuck in the syscall
without noticing the events just happened

Signed-off-by: Yousong Zhou <yszhou4tech at gmail.com>
---
 uloop-epoll.c  |  2 +-
 uloop-kqueue.c |  2 +-
 uloop.c        | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 63 insertions(+), 6 deletions(-)

diff --git a/uloop-epoll.c b/uloop-epoll.c
index bb652fd..6014bea 100644
--- a/uloop-epoll.c
+++ b/uloop-epoll.c
@@ -23,7 +23,7 @@
 #define EPOLLRDHUP 0x2000
 #endif
 
-int uloop_init(void)
+static int uloop_init_pollfd(void)
 {
 	if (poll_fd >= 0)
 		return 0;
diff --git a/uloop-kqueue.c b/uloop-kqueue.c
index 0cb1c14..ba5595b 100644
--- a/uloop-kqueue.c
+++ b/uloop-kqueue.c
@@ -15,7 +15,7 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
-int uloop_init(void)
+static int uloop_init_pollfd(void)
 {
 	struct timespec timeout = { 0, 0 };
 	struct kevent ev = {};
diff --git a/uloop.c b/uloop.c
index cd3de85..b59c31f 100644
--- a/uloop.c
+++ b/uloop.c
@@ -63,6 +63,11 @@ static bool do_sigchld = false;
 static struct uloop_fd_event cur_fds[ULOOP_MAX_EVENTS];
 static int cur_fd, cur_nfds;
 
+static int waker_pipe[2] = {-1, -1};
+static struct uloop_fd waker_fd;
+
+int uloop_fd_add(struct uloop_fd *sock, unsigned int flags);
+
 #ifdef USE_KQUEUE
 #include "uloop-kqueue.c"
 #endif
@@ -71,6 +76,45 @@ static int cur_fd, cur_nfds;
 #include "uloop-epoll.c"
 #endif
 
+static void waker_consume(struct uloop_fd *fd, unsigned int events)
+{
+	char buf[4];
+
+	while (read(fd->fd, buf, 4) > 0)
+		;
+}
+
+static int waker_init(void)
+{
+	if (waker_pipe[0] >= 0 && waker_pipe[1] >= 0)
+		return 0;
+
+	if (pipe(waker_pipe) < 0)
+		return -1;
+
+	fcntl(waker_pipe[0], F_SETFD, fcntl(waker_pipe[0], F_GETFD) | O_NONBLOCK);
+	fcntl(waker_pipe[1], F_SETFD, fcntl(waker_pipe[1], F_GETFD) | O_NONBLOCK);
+
+	waker_fd.fd = waker_pipe[0];
+	waker_fd.cb = waker_consume;
+	uloop_fd_add(&waker_fd, ULOOP_READ);
+
+	return 0;
+}
+
+int uloop_init(void)
+{
+	if (uloop_init_pollfd() < 0)
+		return -1;
+
+	if (waker_init() < 0) {
+		close(poll_fd);
+		poll_fd = -1;
+		return -1;
+	}
+	return 0;
+}
+
 static bool uloop_fd_stack_event(struct uloop_fd *fd, int events)
 {
 	struct uloop_fd_stack *cur;
@@ -330,12 +374,18 @@ static void uloop_handle_processes(void)
 
 static void uloop_handle_sigint(int signo)
 {
+	char buf[1] = {'w'};
+
 	uloop_cancelled = true;
+	write(waker_pipe[1], buf, 1);
 }
 
 static void uloop_sigchld(int signo)
 {
+	char buf[1] = {'w'};
+
 	do_sigchld = true;
+	write(waker_pipe[1], buf, 1);
 }
 
 static void uloop_install_handler(int signum, void (*handler)(int), struct sigaction* old, bool add)
@@ -477,11 +527,18 @@ void uloop_run(void)
 
 void uloop_done(void)
 {
-	if (poll_fd < 0)
-		return;
+	int i;
 
-	close(poll_fd);
-	poll_fd = -1;
+	if (poll_fd >= 0) {
+		close(poll_fd);
+		poll_fd = -1;
+	}
+	for (i = 0; i < ARRAY_SIZE(waker_pipe); i++) {
+		if (waker_pipe[i] >= 0) {
+			close(waker_pipe[i]);
+			waker_pipe[i] = -1;
+		}
+	}
 
 	uloop_clear_timeouts();
 	uloop_clear_processes();
-- 
2.6.4