When a send thread encounters an error (as is the case with yank),
it sets multifd_send_state->exiting and the other threads exit too.
This races with multifd_send_sync_main() which now hangs at
qemu_sem_wait(&p->sem_sync) in multifd_send_sync_main() line 647
as it waits for threads that have exited.

Fix this by kicking the semaphores when exiting the send threads.

I encountered this hang when stress testing the colo unit test,
though I was unable to write a migration test to reliably hit this.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Lukas Straub <lukasstraub2@web.de>
---
 migration/multifd.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index 220ed8564960fdabc58e4baa069dd252c8ad293c..7762aab8e0702672d3730f27e9c9ee3b86500f0c 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -772,9 +772,14 @@ out:
         assert(local_err);
         trace_multifd_send_error(p->id);
         multifd_send_error_propagate(local_err);
-        multifd_send_kick_main(p);
     }
 
+    /*
+     * Always kick the main thread: The main thread might wait on this thread
+     * while another thread encounters an error and signals this thread to exit.
+     */
+    multifd_send_kick_main(p);
+
     rcu_unregister_thread();
     trace_multifd_send_thread_end(p->id, p->packets_sent);
 

-- 
2.39.5