netdev - Re: [PATCH] vhost: Use mutex to protect vq

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Date:   Thu, 29 Oct 2020 09:37:17 +0200
From:   Eli Cohen <elic@...dia.com>
To:     Jason Wang <jasowang@...hat.com>
CC:     "Michael S. Tsirkin" <mst@...hat.com>,
        <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>, <lingshan.zhu@...el.com>
Subject: Re: [PATCH] vhost: Use mutex to protect vq_irq setup

On Thu, Oct 29, 2020 at 03:03:24PM +0800, Jason Wang wrote:
> 
> On 2020/10/28 下午10:20, Eli Cohen wrote:
> > Both irq_bypass_register_producer() and irq_bypass_unregister_producer()
> > require process context to run. Change the call context lock from
> > spinlock to mutex to protect the setup process to avoid deadlocks.
> > 
> > Fixes: 265a0ad8731d ("vhost: introduce vhost_vring_call")
> > Signed-off-by: Eli Cohen <elic@...dia.com>
> 
> 
> Hi Eli:
> 
> During review we spot that the spinlock is not necessary. And it was already
> protected by vq mutex. So it was removed in this commit:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=86e182fe12ee5869022614457037097c70fe2ed1
> 
> Thanks
> 

I see, thanks.

BTW, while testing irq bypassing, I noticed that qemu started crashing
and I fail to boot the VM? Is that a known issue. I checked using
updated master branch of qemu updated yesterday.

Any ideas how to check this further?
Did anyone actually check that irq bypassing works?

> 
> > ---
> >   drivers/vhost/vdpa.c  | 10 +++++-----
> >   drivers/vhost/vhost.c |  6 +++---
> >   drivers/vhost/vhost.h |  3 ++-
> >   3 files changed, 10 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > index be783592fe58..0a744f2b6e76 100644
> > --- a/drivers/vhost/vdpa.c
> > +++ b/drivers/vhost/vdpa.c
> > @@ -98,26 +98,26 @@ static void vhost_vdpa_setup_vq_irq(struct vhost_vdpa *v, u16 qid)
> >   		return;
> >   	irq = ops->get_vq_irq(vdpa, qid);
> > -	spin_lock(&vq->call_ctx.ctx_lock);
> > +	mutex_lock(&vq->call_ctx.ctx_lock);
> >   	irq_bypass_unregister_producer(&vq->call_ctx.producer);
> >   	if (!vq->call_ctx.ctx || irq < 0) {
> > -		spin_unlock(&vq->call_ctx.ctx_lock);
> > +		mutex_unlock(&vq->call_ctx.ctx_lock);
> >   		return;
> >   	}
> >   	vq->call_ctx.producer.token = vq->call_ctx.ctx;
> >   	vq->call_ctx.producer.irq = irq;
> >   	ret = irq_bypass_register_producer(&vq->call_ctx.producer);
> > -	spin_unlock(&vq->call_ctx.ctx_lock);
> > +	mutex_unlock(&vq->call_ctx.ctx_lock);
> >   }
> >   static void vhost_vdpa_unsetup_vq_irq(struct vhost_vdpa *v, u16 qid)
> >   {
> >   	struct vhost_virtqueue *vq = &v->vqs[qid];
> > -	spin_lock(&vq->call_ctx.ctx_lock);
> > +	mutex_lock(&vq->call_ctx.ctx_lock);
> >   	irq_bypass_unregister_producer(&vq->call_ctx.producer);
> > -	spin_unlock(&vq->call_ctx.ctx_lock);
> > +	mutex_unlock(&vq->call_ctx.ctx_lock);
> >   }
> >   static void vhost_vdpa_reset(struct vhost_vdpa *v)
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 9ad45e1d27f0..938239e11455 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -302,7 +302,7 @@ static void vhost_vring_call_reset(struct vhost_vring_call *call_ctx)
> >   {
> >   	call_ctx->ctx = NULL;
> >   	memset(&call_ctx->producer, 0x0, sizeof(struct irq_bypass_producer));
> > -	spin_lock_init(&call_ctx->ctx_lock);
> > +	mutex_init(&call_ctx->ctx_lock);
> >   }
> >   static void vhost_vq_reset(struct vhost_dev *dev,
> > @@ -1650,9 +1650,9 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> >   			break;
> >   		}
> > -		spin_lock(&vq->call_ctx.ctx_lock);
> > +		mutex_lock(&vq->call_ctx.ctx_lock);
> >   		swap(ctx, vq->call_ctx.ctx);
> > -		spin_unlock(&vq->call_ctx.ctx_lock);
> > +		mutex_unlock(&vq->call_ctx.ctx_lock);
> >   		break;
> >   	case VHOST_SET_VRING_ERR:
> >   		if (copy_from_user(&f, argp, sizeof f)) {
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 9032d3c2a9f4..e8855ea04205 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -64,7 +64,8 @@ enum vhost_uaddr_type {
> >   struct vhost_vring_call {
> >   	struct eventfd_ctx *ctx;
> >   	struct irq_bypass_producer producer;
> > -	spinlock_t ctx_lock;
> > +	/* protect vq irq setup */
> > +	struct mutex ctx_lock;
> >   };
> >   /* The virtqueue structure describes a queue attached to a device. */
>