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

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Date:   Thu, 29 Oct 2020 09:50:35 +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 03:39:24PM +0800, Jason Wang wrote:
> 
> On 2020/10/29 下午3:37, Eli Cohen wrote:
> > 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.
> 
> 
> Not known yet.
> 
> 
> > 
> > Any ideas how to check this further?
> 
> 
> I would be helpful if you can paste the calltrace here.
> 

I am not too familiar with qemu. Assuming I am using virsh start to boot
the VM, how can I get the call trace?

> 
> > Did anyone actually check that irq bypassing works?
> 
> 
> Yes, Ling Shan tested it via IFCVF driver.
> 
> Thanks
> 
> 
> > 
> > > > ---
> > > >    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. */
>