From 6baaf82a7742a1de9160146b08ba0cc86b3d4e79 Mon Sep 17 00:00:00 2001

From: Paolo Bonzini <pbonzini@redhat.com>

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 <pbonzini@redhat.com>

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 <jcody@redhat.com>

RH-Acked-by: John Snow <jsnow@redhat.com>

RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>

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

  ><rescue> 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 <rjones@redhat.com>

Tested-by: Richard W.M. Jones <rjones@redhat.com>

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>

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 <pbonzini@redhat.com>

(cherry picked from commit ecbddbb106114f90008024b4e6c3ba1c38d7ca0e)

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 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