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 <[email protected]>
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
