From 0f5319ea25fb0a05827d512859b7c7d23371ac5d Mon Sep 17 00:00:00 2001 From: Igor Mammedov Date: Tue, 5 Jun 2018 16:00:42 +0200 Subject: [PATCH] cli: Don't run early event loop if no --preconfig was specified After 047f7038f586d215 it is possible for event loop to run two times. First time whilst parsing command line options (the idea is to bring up monitor early so that management applications can tweak config before machine is initialized). And the second time is after everything is set up (this is the usual place). In both cases the event loop is called as main_loop_wait(nonblocking = false) which causes the event loop to block until at least one event occurred. Now, consider that somebody (i.e. libvirt) calls us with -daemonize. This operation is split in two steps. The main() calls os_daemonize() which fork()-s and then waits in read() until child notifies it via write(): /qemu.git $ ./x86_64-softmmu/qemu-system-x86_64 -S -daemonize \ -no-user-config -nodefaults -nographic main(): child: os_daemonize(): read(pipe[0]) main_loop(): main_loop_wait(false) os_setup_post(): write(pipe[1]) main_loop(): main_loop_wait(false) Here it can be clearly seen that main() does not exit until an event occurs, but at the same time nobody will touch the monitor socket until their exec("qemu-system-*") finishes. So the whole thing deadlocks. The solution is to not call main_loop_wait() unless --preconfig was specified (in which case caller knows they must connect to the socket before exec() finishes). Patch also fixes hang when -nodefaults option is used, which were causing QEMU hang in the early main_loop_wait() indefinitely by the same means (not calling main_loop_wait() unless --preconfig is present on CLI) Based on From: Michal Privoznik Subject: [PATCH] cli: Don't run early event loop if no --preconfig was specified Message-Id: Fixes: 047f7038f586d215 Signed-off-by: Igor Mammedov Message-Id: <1528207243-268226-2-git-send-email-imammedo@redhat.com> Signed-off-by: Eduardo Habkost --- vl.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/vl.c b/vl.c index 06031715ac..6e34fb348d 100644 --- a/vl.c +++ b/vl.c @@ -1841,7 +1841,7 @@ static void main_loop(void) #ifdef CONFIG_PROFILER int64_t ti; #endif - do { + while (!main_loop_should_exit()) { #ifdef CONFIG_PROFILER ti = profile_getclock(); #endif @@ -1849,7 +1849,7 @@ static void main_loop(void) #ifdef CONFIG_PROFILER dev_time += profile_getclock() - ti; #endif - } while (!main_loop_should_exit()); + } } static void version(void)