Re: [Qemu-devel] [PATCH] blockdev: acquire aio_context for bitmap add/remove

[Eric Blake]

On 2/6/19 11:02 AM, 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.
> 
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1672010
> Reported-By: Aihua Liang <address@hidden>
> Signed-off-by: John Snow <address@hidden>
> ---
>  blockdev.c | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/blockdev.c b/blockdev.c
> index fb18e9c975..ce458de037 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -2820,6 +2820,7 @@ void qmp_block_dirty_bitmap_add(const char *node, const 
> char *name,
>  {
>      BlockDriverState *bs;
>      BdrvDirtyBitmap *bitmap;
> +    AioContext *aio_context = NULL;
>  
>      if (!name || name[0] == '\0') {
>          error_setg(errp, "Bitmap name cannot be empty");
> @@ -2854,10 +2855,12 @@ void qmp_block_dirty_bitmap_add(const char *node, 
> const char *name,
>          disabled = false;
>      }
>  
> -    if (persistent &&
> -        !bdrv_can_store_new_dirty_bitmap(bs, name, granularity, errp))
> -    {
> -        return;
> +    if (persistent) {
> +        aio_context = bdrv_get_aio_context(bs);
> +        aio_context_acquire(aio_context);
> +        if (!bdrv_can_store_new_dirty_bitmap(bs, name, granularity, errp)) {
> +            goto out;
> +        }
>      }
>  
>      bitmap = bdrv_create_dirty_bitmap(bs, granularity, name, errp);
    if (bitmap == NULL) {
        return;
    }

Oops - this early return fails to release the aio context.

> @@ -2870,6 +2873,10 @@ void qmp_block_dirty_bitmap_add(const char *node, 
> const char *name,
>      }
>  
>      bdrv_dirty_bitmap_set_persistance(bitmap, persistent);
> + out:
> +    if (aio_context) {
> +        aio_context_release(aio_context);
> +    }
>  }
>  
>  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);
> +    }
>  }
>  
>  /**
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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