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Re: [Qemu-devel] [PATCH 1/2] migration: Reset rather than destroy main_t

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH 1/2] migration: Reset rather than destroy main_thread_load_event
Date: Fri, 25 Aug 2017 16:28:26 +0100

On 25 August 2017 at 15:19, Dr. David Alan Gilbert (git)
<address@hidden> wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> migration_incoming_state_destroy doesn't really destroy, it cleans up.
> After a loadvm it's called, but the loadvm command can be run twice,
> and so destroying an init-once mutex breaks on the second loadvm.
> Reported-by: Stafford Horne <address@hidden>
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> ---
>  migration/migration.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/migration/migration.c b/migration/migration.c
> index c3fe0ed9ca..a625551ce5 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -167,7 +167,7 @@ void migration_incoming_state_destroy(void)
>          mis->from_src_file = NULL;
>      }
> -    qemu_event_destroy(&mis->main_thread_load_event);
> +    qemu_event_reset(&mis->main_thread_load_event);
>  }

Is it worth doing more here? For instance:
 * rename the function to something that better reflects
   what it's doing
 * make sure it actually goes back to the state it's in
   after you first call migration_incoming_get_current()
   (eg setting mis_current.state, zeroing various fields)
 * maybe instead of a "get current state" that does a
   reset if it's not been called before and a "destroy"
   that does cleanup stuff (like telling the source we've
   stopped) and also resetting back to clean state, we
   could structure this with a function that does
   "give me a clean completely reset state" which you
   must call first, then use get_current() purely to
   get the current state with no 'reset on first call'
   semantics, and finally a "complete" function that just
   does the "tell source we've stopped" and close
   resources we no longer need  ?

PS, in migration_incoming_get_current() we do
        mis_current.state = MIGRATION_STATUS_NONE;
        memset(&mis_current, 0, sizeof(MigrationIncomingState));

and the first line there is pointless because the memset
blasts zeroes over it anyway.

-- PMM

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