Re: [PATCH v5 07/36] migration: postcopy_ram_listen_thread() should take BQL for some calls

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 19, 2025 at 09:33:49PM +0100, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> All callers to migration_incoming_state_destroy() other than
> postcopy_ram_listen_thread() do this call with BQL held.
> 
> Since migration_incoming_state_destroy() ultimately calls "load_cleanup"
> SaveVMHandlers and it will soon call BQL-sensitive code it makes sense
> to always call that function under BQL rather than to have it deal with
> both cases (with BQL and without BQL).
> Add the necessary bql_lock() and bql_unlock() to
> postcopy_ram_listen_thread().

We can do that, but let's be explicit on what needs BQL to be taken.

Could you add an assertion in migration_incoming_state_destroy() on
bql_locked(), then add a rich comment above it listing what needs the BQL?
We may consider dropping it some day when it's not needed.

Thanks,

> 
> qemu_loadvm_state_main() in postcopy_ram_listen_thread() could call
> "load_state" SaveVMHandlers that are expecting BQL to be held.
> 
> In principle, the only devices that should be arriving on migration
> channel serviced by postcopy_ram_listen_thread() are those that are
> postcopiable and whose load handlers are safe to be called without BQL
> being held.
> 
> But nothing currently prevents the source from sending data for "unsafe"
> devices which would cause trouble there.
> Add a TODO comment there so it's clear that it would be good to improve
> handling of such (erroneous) case in the future.
> 
> 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 7c1aa8ad7b9d..3e86b572cfa8 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1986,6 +1986,8 @@ static void *postcopy_ram_listen_thread(void *opaque)
>       * in qemu_file, and thus we must be blocking now.
>       */
>      qemu_file_set_blocking(f, true);
> +
> +    /* TODO: sanity check that only postcopiable data will be loaded here */
>      load_res = qemu_loadvm_state_main(f, mis);
>  
>      /*
> @@ -2046,7 +2048,9 @@ 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;
> 

-- 
Peter Xu