[PATCH] migration/postcopy: Release fd before going into 'postcopy-pause'

[Peter Xu <peterx@redhat.com>]

Logically below race could trigger with the old code:

          test program                        migration thread
          ------------                        ----------------
       wait_until('postcopy-pause')
                                          postcopy_pause()
                                            set_state('postcopy-pause')
       do_postcopy_recover()
         arm s->to_dst_file with new fd
                                            release s->to_dst_file [1]

Here [1] could have released the just-installed recoverying channel.  Then the
migration could hang without really resuming.

Instead, it should be very safe to release the fd before setting the state into
'postcopy-pause', because there's no reason for any other thread to touch it
during 'postcopy-active'.

Dave reported a very rare postcopy recovery hang that the migration-test
program waited for the migration to complete in migrate_postcopy_complete().
We do suspect it's the same thing that we're gonna fix here.  Hard to tell.
However since we've noticed this, fix this irrelevant of the hang report.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index aca7fdcd0b..e1b06e8a2a 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3137,9 +3137,6 @@ static MigThrError postcopy_pause(MigrationState *s)
     while (true) {
         QEMUFile *file;
 
-        migrate_set_state(&s->state, s->state,
-                          MIGRATION_STATUS_POSTCOPY_PAUSED);
-
         /* Current channel is possibly broken. Release it. */
         assert(s->to_dst_file);
         qemu_mutex_lock(&s->qemu_file_lock);
@@ -3150,6 +3147,9 @@ static MigThrError postcopy_pause(MigrationState *s)
         qemu_file_shutdown(file);
         qemu_fclose(file);
 
+        migrate_set_state(&s->state, s->state,
+                          MIGRATION_STATUS_POSTCOPY_PAUSED);
+
         error_report("Detected IO failure for postcopy. "
                      "Migration paused.");
 
-- 
2.26.2