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

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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>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201021212721.440373-6-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
This commit is contained in:
Peter Xu 2020-10-21 17:27:20 -04:00 committed by Dr. David Alan Gilbert
parent 0c26781c09
commit d246ea5039
1 changed files with 3 additions and 3 deletions
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6
migration/migration.c
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@ -3178,9 +3178,6 @@ 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);
@ -3191,6 +3188,9 @@ 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.");