Re: [Qemu-devel] [PATCH v9 3/8] migration: use bitmap_mutex in migration_bitmap_clear_dirty

[Peter Xu <peterx@redhat.com>]

On Thu, Nov 15, 2018 at 06:07:59PM +0800, Wei Wang wrote:
> The bitmap mutex is used to synchronize threads to update the dirty
> bitmap and the migration_dirty_pages counter. For example, the free
> page optimization clears bits of free pages from the bitmap in an
> iothread context. 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>
> CC: Peter Xu <peterx@redhat.com>
> ---
>  migration/ram.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 7e7deec..ef69dbe 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1556,11 +1556,14 @@ 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;
>  }

It seems fine to me, but have you thought about
test_and_clear_bit_atomic()?  Note that we just had
test_and_set_bit_atomic() a few months ago.

And not related to this patch: I'm unclear on why we have had
bitmap_mutex before, since it seems unnecessary.

Regards,

-- 
Peter Xu