Re: [PATCH] block: temporarily hold the new AioContext of bs_top in bdrv_append()

[Kevin Wolf <kwolf@redhat.com>]

Am 14.02.2023 um 11:51 hat Stefano Garzarella geschrieben:
> bdrv_append() is called with bs_top AioContext held, but
> bdrv_attach_child_noperm() could change the AioContext of bs_top.
> 
> bdrv_replace_node_noperm() calls bdrv_drained_begin() starting from
> commit 2398747128 ("block: Don't poll in bdrv_replace_child_noperm()").
> bdrv_drained_begin() can call BDRV_POLL_WHILE that assumes the new lock
> is taken, so let's temporarily hold the new AioContext to prevent QEMU
> from failing in BDRV_POLL_WHILE when it tries to release the wrong
> AioContext.
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2168209
> Reported-by: Aihua Liang <aliang@redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> I'm not sure whether to use the following Fixes tag. That commit added the
> calls to bdrv_drained_begin() in bdrv_replace_node_noperm(), but maybe the
> problem was pre-existing.
> 
> Fixes: 2398747128 ("block: Don't poll in bdrv_replace_child_noperm()")
> 
> Note: a local reproducer is attached in the BZ, it is based on the Aihua Liang
> report and it hits the issue with a 20% ratio.
> ---
>  block.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/block.c b/block.c
> index aa9062f2c1..0e2bc11e0b 100644
> --- a/block.c
> +++ b/block.c
> @@ -5266,6 +5266,8 @@ int bdrv_drop_filter(BlockDriverState *bs, Error **errp)
>   * child.
>   *
>   * This function does not create any image files.
> + *
> + * The caller must hold the AioContext lock for @bs_top.
>   */
>  int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
>                  Error **errp)
> @@ -5273,11 +5275,14 @@ int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
>      int ret;
>      BdrvChild *child;
>      Transaction *tran = tran_new();
> +    AioContext *old_context, *new_context;
>  
>      GLOBAL_STATE_CODE();
>  
>      assert(!bs_new->backing);
>  
> +    old_context = bdrv_get_aio_context(bs_top);
> +
>      child = bdrv_attach_child_noperm(bs_new, bs_top, "backing",
>                                       &child_of_bds, bdrv_backing_role(bs_new),
>                                       tran, errp);
> @@ -5286,11 +5291,29 @@ int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
>          goto out;
>      }
>  
> +    /*
> +     * bdrv_attach_child_noperm could change the AioContext of bs_top.
> +     * bdrv_replace_node_noperm calls bdrv_drained_begin, so let's temporarily
> +     * hold the new AioContext, since bdrv_drained_begin calls BDRV_POLL_WHILE
> +     * that assumes the new lock is taken.
> +     */
> +    new_context = bdrv_get_aio_context(bs_top);
> +
> +    if (old_context != new_context) {
> +        aio_context_release(old_context);
> +        aio_context_acquire(new_context);
> +    }
> +
>      ret = bdrv_replace_node_noperm(bs_top, bs_new, true, tran, errp);
>      if (ret < 0) {
>          goto out;

If we take the error path, we return with new_context locked instead of
old_context now.

>      }
>  
> +    if (old_context != new_context) {
> +        aio_context_release(new_context);
> +        aio_context_acquire(old_context);
> +    }
> +
>      ret = bdrv_refresh_perms(bs_new, tran, errp);
>  out:
>      tran_finalize(tran, ret);

Strictly speaking, don't we need to hold the lock across
tran_finalize(), too? It completes the bdrv_replace_node_noperm() call
you covered above.

Maybe bdrv_refresh_perms() and bdrv_refresh_limits(), too, in fact. We
never clearly defined which functions need the lock and which don't, so
hard to tell. It's really time to get rid of it.

Kevin