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

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Message-ID: <CAJaqyWf5sC38oT2ZkmfR_MkiYWjLsuyF4Z4opSmgS1nG8BZhgw@mail.gmail.com>
Date: Thu, 29 Jan 2026 07:20:43 +0100
From: Eugenio Perez Martin <eperezma@...hat.com>
To: Jason Wang <jasowang@...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 Thu, Jan 29, 2026 at 2:17 AM Jason Wang <jasowang@...hat.com> wrote:
>
> 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.
>

The function vduse_vq_set/get_ready are not in the datapath, but
vduse_vq_kick and vduse_vq_signal_irqfd are.  I'm ok if you want to
switch to vq_mutex if you want though, maybe it's even comparable with
the cost of the ioctls or eventfd signaling.