Re: [PATCH] block: simplify write-threshold and drop write notifiers

[Stefan Hajnoczi]

On Thu, Apr 22, 2021 at 01:09:50AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> @@ -1981,8 +1985,15 @@ bdrv_co_write_req_prepare(BdrvChild *child, int64_t 
> offset, int64_t bytes,
>          } else {
>              assert(child->perm & BLK_PERM_WRITE);
>          }
> -        return notifier_with_return_list_notify(&bs->before_write_notifiers,
> -                                                req);
> +        write_threshold = qatomic_read(&bs->write_threshold_offset);
> +        if (write_threshold > 0 && offset + bytes > write_threshold) {
> +            qapi_event_send_block_write_threshold(
> +                bs->node_name,
> +                offset + bytes - write_threshold,
> +                write_threshold);
> +            qatomic_set(&bs->write_threshold_offset, 0);

It's safer to reset the threshold before emitting the event. That way
there is no race with the QMP client setting a new threshold via
bdrv_write_threshold_is_set(). I guess the race is possible since
qatomic is used and there is no lock.

I like the idea of dropping write notifiers:
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

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