Re: [Qemu-devel] [PATCH v7 2/5] migration: use bitmap_mutex in migration_bitmap_clear_dirty

[Peter Xu <peterx@redhat.com>]

On Tue, Apr 24, 2018 at 02:13:45PM +0800, Wei Wang wrote:
> The bitmap mutex is used to synchronize threads to update the dirty
> bitmap and the migration_dirty_pages counter. This patch makes
> migration_bitmap_clear_dirty update the bitmap and counter under the
> mutex.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
> CC: Juan Quintela <quintela@redhat.com>
> CC: Michael S. Tsirkin <mst@redhat.com>
> ---
>  migration/ram.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 0e90efa..9a72b1a 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -795,11 +795,14 @@ static inline bool migration_bitmap_clear_dirty(RAMState *rs,
>  {
>      bool ret;
>  
> +    qemu_mutex_lock(&rs->bitmap_mutex);
>      ret = test_and_clear_bit(page, rb->bmap);
>  
>      if (ret) {
>          rs->migration_dirty_pages--;
>      }
> +    qemu_mutex_unlock(&rs->bitmap_mutex);
> +
>      return ret;
>  }
>  
> -- 
> 1.8.3.1
> 
> 

Do we need the lock after all?

I see that we introduced this lock due to device hotplug at dd63169766
("migration: extend migration_bitmap", 2015-07-07), however now we
actually don't allow that to happen any more after commit b06424de62
("migration: Disable hotplug/unplug during migration", 2017-04-21).
I'm not sure whether it means that the lock is not needed now.

If so, this patch seems to be unecessary.

Regards,

-- 
Peter Xu