Re: [PATCH v2 1/1] migration: Unregister yank if migration setup fails

[Peter Xu]

On Tue, Jun 29, 2021 at 02:05:23AM -0300, Leonardo Bras wrote:
> Currently, if a qemu instance is started with "-incoming defer" and
> an incorect parameter is passed to "migrate_incoming", it will print the
> expected error and reply with "duplicate yank instance" for any upcoming
> "migrate_incoming" command.
> 
> This renders current qemu process unusable, and requires a new qemu
> process to be started before accepting a migration.
> 
> This is caused by a yank_register_instance() that happens in
> qemu_start_incoming_migration() but is never reverted if any error
> happens.
> 
> Solves this by unregistering the instance if anything goes wrong
> in the function, allowing a new "migrate_incoming" command to be
> accepted.
> 
> Fixes: b5eea99ec2f ("migration: Add yank feature", 2021-01-13)
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1974366
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> 
> ---
> Changes since v1:
>  - Add ERRP_GUARD() at the beginning of the function, so it deals with
>    errp passed as NULL, and does correct error propagation.
> ---
>  migration/migration.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 4228635d18..af0c72609f 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -454,6 +454,7 @@ void migrate_add_address(SocketAddress *address)
>  
>  static void qemu_start_incoming_migration(const char *uri, Error **errp)
>  {
> +    ERRP_GUARD();
>      const char *p = NULL;
>  
>      if (!yank_register_instance(MIGRATION_YANK_INSTANCE, errp)) {
> @@ -474,9 +475,13 @@ static void qemu_start_incoming_migration(const char 
> *uri, Error **errp)
>      } else if (strstart(uri, "fd:", &p)) {
>          fd_start_incoming_migration(p, errp);
>      } else {
> -        yank_unregister_instance(MIGRATION_YANK_INSTANCE);
>          error_setg(errp, "unknown migration protocol: %s", uri);
>      }
> +
> +    if (*errp) {
> +        yank_unregister_instance(MIGRATION_YANK_INSTANCE);
> +    }
> +
>  }
>  
>  static void process_incoming_migration_bh(void *opaque)
> -- 

Leo,

The patch looks great to me, thanks.

Though I found that maybe we need to fix it in another way due to some other
reason out of scope of this issue. The problem is today I encountered another
yank crashing qemu when trying to do postcopy recover twice.

The problem here is both initial incoming migration and postcopy recovery uses
qemu_start_incoming_migration(), while in qmp_migrate_recover() b5eea99ec2f5c
calls unregister before calling qemu_start_incoming_migration().  I believe it
wanted to mitigate the next call to yank_register_instance() to make it work,
but I think that's wrong...

Firstly, if during recover, we should keep the yank instance there, not
"quickly removing and adding it back".  Meanwhile, as I mentioned calling
qmp_migrate_recover() twice with b5eea99ec2f5c will directly crash the dest
qemu because the 1st call of qmp_migrate_recover() will unregister permanently
when channel failed to establish, then the 2nd call of qmp_migrate_recover()
crashes at yank_unregister_instance().

I'll post an alternative fix of this issue (plus another postcopy recovery fix)
to show better on what I meant.

Thanks,

-- 
Peter Xu