Re: [Qemu-devel] [PULL v2 3/3] blockdev: acquire aio_context for bitmap add/remove

[Vladimir Sementsov-Ogievskiy]

Hi!

20.02.2019 21:01, John Snow wrote:
> When bitmaps are persistent, they may incur a disk read or write when bitmaps
> are added or removed. For configurations like virtio-dataplane, failing to
> acquire this lock will abort QEMU when disk IO occurs.
> 
> We used to acquire aio_context as part of the bitmap lookup, so re-introduce
> the lock for just the cases that have an IO penalty. Commit 2119882c removed
> these locks, and I failed to notice this when we committed fd5ae4cc, so this
> has been broken since persistent bitmaps were introduced.
> 
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1672010
> Reported-By: Aihua Liang <address@hidden>
> Signed-off-by: John Snow <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
> Message-id: address@hidden
> Signed-off-by: John Snow <address@hidden>
> ---

[..]

>   void qmp_block_dirty_bitmap_remove(const char *node, const char *name,
> @@ -2878,6 +2885,7 @@ void qmp_block_dirty_bitmap_remove(const char *node, 
> const char *name,
>       BlockDriverState *bs;
>       BdrvDirtyBitmap *bitmap;
>       Error *local_err = NULL;
> +    AioContext *aio_context = NULL;
>   
>       bitmap = block_dirty_bitmap_lookup(node, name, &bs, errp);
>       if (!bitmap || !bs) {
> @@ -2892,14 +2900,20 @@ void qmp_block_dirty_bitmap_remove(const char *node, 
> const char *name,
>       }
>   
>       if (bdrv_dirty_bitmap_get_persistance(bitmap)) {
> +        aio_context = bdrv_get_aio_context(bs);
> +        aio_context_acquire(aio_context);
>           bdrv_remove_persistent_dirty_bitmap(bs, name, &local_err);
>           if (local_err != NULL) {
>               error_propagate(errp, local_err);
> -            return;
> +            goto out;
>           }
>       }
>   
>       bdrv_release_dirty_bitmap(bs, bitmap);
> + out:
> +    if (aio_context) {
> +        aio_context_release(aio_context);
> +    }
>   }
>   
>   /**
> 

A bit late, but I have a question:

Why did you include bdrv_release_dirty_bitmap call into context-acquired 
section? As I can
understand from commit message, it's not actually needed?

-- 
Best regards,
Vladimir