[PATCH] migration: Wait on preempt channel in preempt thread

[Peter Xu <peterx@redhat.com>]

QEMU main thread will wait until dest preempt channel established during
processing the LISTEN command (within the whole postcopy PACKAGED data), by
waiting on the semaphore postcopy_qemufile_dst_done.

That's racy, because it's possible that the dest QEMU main thread hasn't
yet accept()ed the new connection when processing the LISTEN event.  The
sem_wait() will yield the main thread without being able to run anything
else including the accept() of the new socket, which can cause deadlock
within the main thread.

To avoid the race, move the "wait channel" from main thread to the preempt
thread right at the start.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: 5655aab079 ("migration: Postpone postcopy preempt channel to be after main")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---

PS: This patch is supposed to fix the test breakage reported here:

https://lore.kernel.org/r/ZBBIaX+cZD5Ud2wQ@work-vm
---
 migration/postcopy-ram.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index f54f44d899..41c0713650 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -1197,11 +1197,6 @@ int postcopy_ram_incoming_setup(MigrationIncomingState *mis)
     }
 
     if (migrate_postcopy_preempt()) {
-        /*
-         * The preempt channel is established in asynchronous way.  Wait
-         * for its completion.
-         */
-        qemu_sem_wait(&mis->postcopy_qemufile_dst_done);
         /*
          * This thread needs to be created after the temp pages because
          * it'll fetch RAM_CHANNEL_POSTCOPY PostcopyTmpPage immediately.
@@ -1668,6 +1663,12 @@ void *postcopy_preempt_thread(void *opaque)
 
     qemu_sem_post(&mis->thread_sync_sem);
 
+    /*
+     * The preempt channel is established in asynchronous way.  Wait
+     * for its completion.
+     */
+    qemu_sem_wait(&mis->postcopy_qemufile_dst_done);
+
     /* Sending RAM_SAVE_FLAG_EOS to terminate this thread */
     qemu_mutex_lock(&mis->postcopy_prio_thread_mutex);
     while (1) {
-- 
2.39.1