[PATCH] system/runstate: Fix regression, clarify BQL status of exit notifiers

[Phil Dennis-Jordan]

By changing the way the main QEMU event loop is invoked, I inadvertently
changed the BQL status of exit notifiers: some of them implicitly
assumed they would be called with the BQL held; the BQL is however
not held during the exit(status) call in qemu_default_main().

Instead of attempting to ensuring we always call exit() from the BQL -
including any transitive calls - this change adds a BQL lock guard to
qemu_run_exit_notifiers, ensuring the BQL will always be held in the
exit notifiers.

Additionally, the BQL promise is now documented at the
qemu_{add,remove}_exit_notifier() declarations.

Fixes: f5ab12caba4f ("ui & main loop: Redesign of system-specific main
thread event handling")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2771
Reported-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
---
 include/system/system.h | 1 +
 system/runstate.c       | 1 +
 2 files changed, 2 insertions(+)

diff --git a/include/system/system.h b/include/system/system.h
index 5364ad4f27..0cbb43ec30 100644
--- a/include/system/system.h
+++ b/include/system/system.h
@@ -15,6 +15,7 @@ extern bool qemu_uuid_set;
 
 const char *qemu_get_vm_name(void);
 
+/* Exit notifiers will run with BQL held. */
 void qemu_add_exit_notifier(Notifier *notify);
 void qemu_remove_exit_notifier(Notifier *notify);
 
diff --git a/system/runstate.c b/system/runstate.c
index 3a8fe866bc..272801d307 100644
--- a/system/runstate.c
+++ b/system/runstate.c
@@ -850,6 +850,7 @@ void qemu_remove_exit_notifier(Notifier *notify)
 
 static void qemu_run_exit_notifiers(void)
 {
+    BQL_LOCK_GUARD();
     notifier_list_notify(&exit_notifiers, NULL);
 }
 
-- 
2.39.5 (Apple Git-154)