From 6baaf82a7742a1de9160146b08ba0cc86b3d4e79 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Wed, 10 Jan 2018 17:02:21 +0100 Subject: [PATCH 2/2] main-loop: Acquire main_context lock around os_host_main_loop_wait. RH-Author: Paolo Bonzini Message-id: <20180110170221.28975-1-pbonzini@redhat.com> Patchwork-id: 78541 O-Subject: [RHEL7.5 qemu-kvm PATCH] main-loop: Acquire main_context lock around os_host_main_loop_wait. Bugzilla: 1473536 RH-Acked-by: Jeffrey Cody RH-Acked-by: John Snow RH-Acked-by: Miroslav Rezanina Bugzilla: 1473536 Brew build: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=14912977 When running virt-rescue the serial console hangs from time to time. Virt-rescue runs an ordinary Linux kernel "appliance", but there is only a single idle process running inside, so the qemu main loop is largely idle. With virt-rescue >= 1.37 you may be able to observe the hang by doing: $ virt-rescue -e ^] --scratch > while true; do ls -l /usr/bin; done The hang in virt-rescue can be resolved by pressing a key on the serial console. Possibly with the same root cause, we also observed hangs during very early boot of regular Linux VMs with a serial console. Those hangs are extremely rare, but you may be able to observe them by running this command on baremetal for a sufficiently long time: $ while libguestfs-test-tool -t 60 >& /tmp/log ; do echo -n . ; done (Check in /tmp/log that the failure was caused by a hang during early boot, and not some other reason) During investigation of this bug, Paolo Bonzini wrote: > glib is expecting QEMU to use g_main_context_acquire around accesses to > GMainContext. However QEMU is not doing that, instead it is taking its > own mutex. So we should add g_main_context_acquire and > g_main_context_release in the two implementations of > os_host_main_loop_wait; these should undo the effect of Frediano's > glib patch. This patch exactly implements Paolo's suggestion in that paragraph. This fixes the serial console hang in my testing, across 3 different physical machines (AMD, Intel Core i7 and Intel Xeon), over many hours of automated testing. I wasn't able to reproduce the early boot hangs (but as noted above, these are extremely rare in any case). Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1435432 Reported-by: Richard W.M. Jones Tested-by: Richard W.M. Jones Signed-off-by: Richard W.M. Jones Message-Id: <20170331205133.23906-1-rjones@redhat.com> [Paolo: this is actually a glib bug: recent glib versions are also expecting g_main_context_acquire around g_poll---but that is not documented and probably not even intended]. Signed-off-by: Paolo Bonzini (cherry picked from commit ecbddbb106114f90008024b4e6c3ba1c38d7ca0e) Signed-off-by: Miroslav Rezanina --- main-loop.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/main-loop.c b/main-loop.c index cf36645..a93d37b 100644 --- a/main-loop.c +++ b/main-loop.c @@ -192,9 +192,12 @@ static void glib_pollfds_poll(void) static int os_host_main_loop_wait(uint32_t timeout) { + GMainContext *context = g_main_context_default(); int ret; static int spin_counter; + g_main_context_acquire(context); + glib_pollfds_fill(&timeout); /* If the I/O thread is very busy or we are incorrectly busy waiting in @@ -230,6 +233,9 @@ static int os_host_main_loop_wait(uint32_t timeout) } glib_pollfds_poll(); + + g_main_context_release(context); + return ret; } #else @@ -385,12 +391,15 @@ static int os_host_main_loop_wait(uint32_t timeout) fd_set rfds, wfds, xfds; int nfds; + g_main_context_acquire(context); + /* XXX: need to suppress polling by better using win32 events */ ret = 0; for (pe = first_polling_entry; pe != NULL; pe = pe->next) { ret |= pe->func(pe->opaque); } if (ret != 0) { + g_main_context_release(context); return ret; } @@ -440,6 +449,8 @@ static int os_host_main_loop_wait(uint32_t timeout) g_main_context_dispatch(context); } + g_main_context_release(context); + return select_ret || g_poll_ret; } #endif -- 1.8.3.1