linux-kernel - Re: [PATCH] vhost/net: Replace wait_queue with completion in ubufs reference

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Message-ID: <20250718090725.2352-1-hdanton@sina.com>
Date: Fri, 18 Jul 2025 17:07:24 +0800
From: Hillf Danton <hdanton@...a.com>
To: Nikolay Kuratov <kniv@...dex-team.ru>
Cc: linux-kernel@...r.kernel.org,
	virtualization@...ts.linux.dev,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Lei Yang <leiyang@...hat.com>,
	Andrey Ryabinin <arbn@...dex-team.com>,
	Andrey Smetanin <asmetanin@...dex-team.ru>
Subject: Re: [PATCH] vhost/net: Replace wait_queue with completion in ubufs reference

On Wed, 16 Jul 2025 19:22:43 +0300 Nikolay Kuratov wrote:
>  drivers/vhost/net.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 7cbfc7d718b3..454d179fffeb 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -94,7 +94,7 @@ struct vhost_net_ubuf_ref {
>  	 * >1: outstanding ubufs
>  	 */
>  	atomic_t refcount;
> -	wait_queue_head_t wait;
> +	struct completion wait;
>  	struct vhost_virtqueue *vq;
>  };
>  
> @@ -240,7 +240,7 @@ vhost_net_ubuf_alloc(struct vhost_virtqueue *vq, bool zcopy)
>  	if (!ubufs)
>  		return ERR_PTR(-ENOMEM);
>  	atomic_set(&ubufs->refcount, 1);
> -	init_waitqueue_head(&ubufs->wait);
> +	init_completion(&ubufs->wait);
>  	ubufs->vq = vq;
>  	return ubufs;
>  }
> @@ -249,14 +249,15 @@ static int vhost_net_ubuf_put(struct vhost_net_ubuf_ref *ubufs)
>  {
>  	int r = atomic_sub_return(1, &ubufs->refcount);
>  	if (unlikely(!r))
> -		wake_up(&ubufs->wait);
> +		complete_all(&ubufs->wait);
>  	return r;
>  }
>  
>  static void vhost_net_ubuf_put_and_wait(struct vhost_net_ubuf_ref *ubufs)
>  {
>  	vhost_net_ubuf_put(ubufs);
> -	wait_event(ubufs->wait, !atomic_read(&ubufs->refcount));
> +	wait_for_completion(&ubufs->wait);
> +	reinit_completion(&ubufs->wait);

In the case of 5 waiters for example, after the first waiter reinitializes
the completion, the 3rd waiter misses the wakeup, no?

>  }
>  
>  static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
> -- 
> 2.34.1