Re: [PATCH v2 6/9] migration: Don't set FAILED state when cancelling

[Peter Xu]

On Tue, Feb 11, 2025 at 12:01:33PM -0300, Fabiano Rosas wrote:
> It's possible that the migration is cancelled during
> migration_switchover_start(). In that case, don't set the migration
> state FAILED in migration_completion().
> 
> Fixes: 3dde8fdbad ("migration: Merge precopy/postcopy on switchover start")
> Signed-off-by: Fabiano Rosas <faro...@suse.de>

I remember I paid some attention on this one when working on the commit,
where it has:

static bool migration_switchover_prepare(MigrationState *s)
{
    /* Concurrent cancellation?  Quit */
    if (s->state == MIGRATION_STATUS_CANCELLING) {   <================= [1]
        return false;
    }
    ...
    bql_unlock();

    qemu_sem_wait(&s->pause_sem);

    bql_lock();
    /*
     * After BQL released and retaken, the state can be CANCELLING if it
     * happend during sem_wait().. Only change the state if it's still
     * pre-switchover.
     */
    migrate_set_state(&s->state, MIGRATION_STATUS_PRE_SWITCHOVER, <====== [2]
                      MIGRATION_STATUS_DEVICE);

    return s->state == MIGRATION_STATUS_DEVICE;
}

So when holding BQL logically it can't change to CANCELLING, it'll check
first [1] making sure no prior CANCELLING.  Then after release and retake
BQL it'll check again [2] (see the comment above [2], it's done by passing
in explicit old_state to not change it if it's CANCELLING).

Any hint on how this could be triggered?

OTOH, when looking at this.. I seem to have found a bug indeed (which could
be another?), where I may have forgot to touch up the old_state in
migrate_set_state() after switching to always use DEVICE..

diff --git a/migration/migration.c b/migration/migration.c
index 74c50cc72c..513e5955cc 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2793,8 +2793,9 @@ static int postcopy_start(MigrationState *ms, Error 
**errp)
 fail_closefb:
     qemu_fclose(fb);
 fail:
-    migrate_set_state(&ms->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
-                          MIGRATION_STATUS_FAILED);
+    if (ms->state != MIGRATION_STATUS_CANCELLING) {
+        migrate_set_state(&ms->state, ms->state, MIGRATION_STATUS_FAILED);
+    }
     migration_block_activate(NULL);
     migration_call_notifiers(ms, MIG_EVENT_PRECOPY_FAILED, NULL);
     bql_unlock();

I'm not sure whether it's relevant to what you hit, though.. since you're
looking at this, I'd rely on you help figuring it out before I do.. :)

> ---
>  migration/migration.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 375de6d460..5dc43bcdc0 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2986,7 +2986,9 @@ fail:
>          error_free(local_err);
>      }
>  
> -    migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
> +    if (s->state != MIGRATION_STATUS_CANCELLING) {
> +        migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
> +    }
>  }
>  
>  /**
> -- 
> 2.35.3
> 

-- 
Peter Xu