[Qemu-devel] [PATCH] main-loop: Don't lock starve io-threads when main_loop_tlg has pending events

[Hans de Goede <hdegoede@redhat.com>]

I noticed today that current qemu master would hang as soon as Xorg starts in
the guest when using qxl + a Linux guest. This message would be printed:
main-loop: WARNING: I/O thread spun for 1000 iterations

And from then on the guest hangs and qemu consumes 100% cpu, bisecting pointed
out commit 7b595f35d89d73bc69c35bf3980a89c420e8a44b:
"aio / timers: Convert mainloop to use timeout"

After looking at that commit I had a hunch the problem might be blocking
main_loop_wait calls being turned into non-blocking ones (and thus never
releasing the io-lock), a debug printf confirmed this was happening at
the moment of the hang, so I wrote this patch which fixes the hang for me
and seems like a good idea in general.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 main-loop.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/main-loop.c b/main-loop.c
index c3c9c28..921c939 100644
--- a/main-loop.c
+++ b/main-loop.c
@@ -480,6 +480,11 @@ int main_loop_wait(int nonblocking)
                                       timerlistgroup_deadline_ns(
                                           &main_loop_tlg));
 
+    /* When not non-blocking always allow io-threads to acquire the lock */
+    if (timeout != 0 && timeout_ns == 0) {
+        timeout_ns = 1;
+    }
+
     ret = os_host_main_loop_wait(timeout_ns);
     qemu_iohandler_poll(gpollfds, ret);
 #ifdef CONFIG_SLIRP
-- 
1.8.3.1