From cc04cfb41bece6ec239f57d83822286b507f4482 Mon Sep 17 00:00:00 2001 From: Uli Schlachter Date: Mon, 18 May 2015 21:40:34 +0200 Subject: [PATCH] send_fds(): Make sure no other thread interrupts us Two threads trying to send fds at the same time could interfere. To guarantee a correct ordering, we have to use correct locking. The code in send_fds() missed one case: If there was another thread already writing requests, we slept on the "done with writing" condition variable (c->out.cond). This would allow other threads to re-acquire the iolock before us and could cause fds to be sent out of order. To fix this, at the beginning of send_fds() we now make sure that no other thread is already writing requests. This is what prepare_socket_request() does. Additionally, it gets the socket back in case xcb_take_socket() was called, which is a good thing, too, since fds are only sent with corresponding requests. --- src/xcb_out.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/src/xcb_out.c b/src/xcb_out.c index 722463e..ec132fc 100644 --- a/src/xcb_out.c +++ b/src/xcb_out.c @@ -186,6 +186,15 @@ static void close_fds(int *fds, unsigned int num_fds) static void send_fds(xcb_connection_t *c, int *fds, unsigned int num_fds) { #if HAVE_SENDMSG + /* Calling _xcb_out_flush_to() can drop the iolock and wait on a condition + * variable if another thread is currently writing (c->out.writing > 0). + * This call waits for writers to be done and thus _xcb_out_flush_to() will + * do the work itself (in which case we are a writer and + * prepare_socket_request() will wait for us to be done if another threads + * tries to send fds, too). Thanks to this, we can atomically write out FDs. + */ + prepare_socket_request(c); + while (num_fds > 0) { /* FIXME: This will busy-loop when XCB_MAX_PASS_FD fds are sent at once */ while (c->out.out_fd.nfd == XCB_MAX_PASS_FD && !c->has_error) {