netdev - Re: [PATCH v9 16/17] vduse: Introduce VDUSE

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Date:   Wed, 14 Jul 2021 01:54:58 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Xie Yongji <xieyongji@...edance.com>, stefanha@...hat.com,
        sgarzare@...hat.com, parav@...dia.com, hch@...radead.org,
        christian.brauner@...onical.com, rdunlap@...radead.org,
        willy@...radead.org, viro@...iv.linux.org.uk, axboe@...nel.dk,
        bcrl@...ck.org, corbet@....net, mika.penttila@...tfour.com,
        dan.carpenter@...cle.com, joro@...tes.org,
        gregkh@...uxfoundation.org, zhe.he@...driver.com,
        xiaodong.liu@...el.com, songmuchun@...edance.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 16/17] vduse: Introduce VDUSE - vDPA Device in
 Userspace

On Wed, Jul 14, 2021 at 01:45:39PM +0800, Jason Wang wrote:
> > +static int vduse_dev_msg_sync(struct vduse_dev *dev,
> > +			      struct vduse_dev_msg *msg)
> > +{
> > +	int ret;
> > +
> > +	init_waitqueue_head(&msg->waitq);
> > +	spin_lock(&dev->msg_lock);
> > +	msg->req.request_id = dev->msg_unique++;
> > +	vduse_enqueue_msg(&dev->send_list, msg);
> > +	wake_up(&dev->waitq);
> > +	spin_unlock(&dev->msg_lock);
> > +
> > +	wait_event_killable_timeout(msg->waitq, msg->completed,
> > +				    VDUSE_REQUEST_TIMEOUT * HZ);
> > +	spin_lock(&dev->msg_lock);
> > +	if (!msg->completed) {
> > +		list_del(&msg->list);
> > +		msg->resp.result = VDUSE_REQ_RESULT_FAILED;
> > +	}
> > +	ret = (msg->resp.result == VDUSE_REQ_RESULT_OK) ? 0 : -EIO;
> 
> 
> I think we should mark the device as malfunction when there is a timeout and
> forbid any userspace operations except for the destroy aftwards for safety.

This looks like if one tried to run gdb on the program the behaviour
will change completely because kernel wants it to respond within
specific time. Looks like a receipe for heisenbugs.

Let's not build interfaces with arbitrary timeouts like that.
Interruptible wait exists for this very reason. Let userspace have its
own code to set and use timers. This does mean that userspace will
likely have to change a bit to support this driver, such is life.

-- 
MST