Re: [PATCH v4 09/33] migration: postcopy_ram_listen_thread() needs to take BQL for some calls

["Dr. David Alan Gilbert" <dave@treblig.org>]

* Maciej S. Szmigiero (mail@maciej.szmigiero.name) wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> postcopy_ram_listen_thread() is a free running thread, so it needs to
> take BQL around function calls to migration methods requiring BQL.
> 
> qemu_loadvm_state_main() needs BQL held since it ultimately calls
> "load_state" SaveVMHandlers.
> 
> migration_incoming_state_destroy() needs BQL held since it ultimately calls
> "load_cleanup" SaveVMHandlers.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>  migration/savevm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index b0b74140daea..0ceea9638cc1 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2013,7 +2013,9 @@ static void *postcopy_ram_listen_thread(void *opaque)
>       * in qemu_file, and thus we must be blocking now.
>       */
>      qemu_file_set_blocking(f, true);
> +    bql_lock();
>      load_res = qemu_loadvm_state_main(f, mis);
> +    bql_unlock();

Doesn't that leave that held for a heck of a long time?
That RAM loading has to happen in parallel with the loading of
devices doesn't it - especially if one of the devices
being loaded touches RAM.

(I wish this series had a description in the cover letter!)

Dave


>      /*
>       * This is tricky, but, mis->from_src_file can change after it
> @@ -2073,7 +2075,9 @@ static void *postcopy_ram_listen_thread(void *opaque)
>       * (If something broke then qemu will have to exit anyway since it's
>       * got a bad migration state).
>       */
> +    bql_lock();
>      migration_incoming_state_destroy();
> +    bql_unlock();
>  
>      rcu_unregister_thread();
>      mis->have_listen_thread = false;
> 
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
\        dave @ treblig.org |                               | In Hex /
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