Re: [PATCH v3 5/8] block/write-threshold: don't use aio context lock

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/05/21 11:06, Vladimir Sementsov-Ogievskiy wrote:
>   void bdrv_write_threshold_check_write(BlockDriverState *bs, int64_t offset,
>                                         int64_t bytes)
>   {
>       int64_t end = offset + bytes;
> -    uint64_t wtr = bs->write_threshold_offset;
> +    uint64_t wtr;
>   
> -    if (wtr > 0 && end > wtr) {
> -        qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr);
> +retry:
> +    wtr = bdrv_write_threshold_get(bs);
> +    if (wtr == 0 || wtr >= end) {
> +        return;
> +    }
>   
> -        /* autodisable to avoid flooding the monitor */
> -        bdrv_write_threshold_set(bs, 0);
> +    /* autodisable to avoid flooding the monitor */
> +    if (qatomic_cmpxchg(&bs->write_threshold_offset, wtr, 0) != wtr) {
> +        /* bs->write_threshold_offset changed in parallel */
> +        goto retry;
>       }
> +
> +    /* We have cleared bs->write_threshold_offset, so let's send event */
> +    qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr);
>   }
> 

This has the problem that 64-bit atomics are not always possible on 
32-bit builds.  We can use a spinlock (and probably just drop this patch 
for now).

Paolo