linux-kernel - Re: [PATCH 1/6] vduse: ensure vq->ready access is smp safe

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Message-ID: <CACGkMEuDPgkEVzpu0c+oKvOqgaSR5=CnHMe9iZDi_PFQan0m6g@mail.gmail.com>
Date: Thu, 29 Jan 2026 09:16:50 +0800
From: Jason Wang <jasowang@...hat.com>
To: Eugenio Pérez <eperezma@...hat.com>
Cc: "Michael S . Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Cindy Lu <lulu@...hat.com>, Laurent Vivier <lvivier@...hat.com>, 
	Stefano Garzarella <sgarzare@...hat.com>, linux-kernel@...r.kernel.org, 
	Maxime Coquelin <mcoqueli@...hat.com>, Yongji Xie <xieyongji@...edance.com>, 
	virtualization@...ts.linux.dev
Subject: Re: [PATCH 1/6] vduse: ensure vq->ready access is smp safe

On Wed, Jan 28, 2026 at 8:45 PM Eugenio Pérez <eperezma@...hat.com> wrote:
>
> The vduse_vdpa_set_vq_ready can be called in the lifetime of the device
> well after initial setup, and the device can read it afterwards.
>
> Ensure that reads and writes to vq->ready are SMP safe so that the
> caller can trust that virtqueue kicks and calls behave as expected
> immediately after the operation returns.
>
> Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> ---
>  drivers/vdpa/vdpa_user/vduse_dev.c | 34 +++++++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> index 73d1d517dc6c..a4963aaf9332 100644
> --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> @@ -460,6 +460,24 @@ static __poll_t vduse_dev_poll(struct file *file, poll_table *wait)
>         return mask;
>  }
>
> +static bool vduse_vq_get_ready(const struct vduse_virtqueue *vq)
> +{
> +       /*
> +        * Paired with vduse_vq_set_ready smp_store, as the driver may modify
> +        * it while the VDUSE instance is reading it.
> +        */
> +       return smp_load_acquire(&vq->ready);
> +}
> +
> +static void vduse_vq_set_ready(struct vduse_virtqueue *vq, bool ready)
> +{
> +       /*
> +        * Paired with vduse_vq_get_ready smp_load, as the driver may modify
> +        * it while the VDUSE instance is reading it.
> +        */
> +       smp_store_release(&vq->ready, ready);

Assuming this is not used in the datapath, I wonder if we can simply
use vq_lock mutex.

Thanks