Re: [PATCH V4 6/9] virtio-ccw: implement synchronize_cbs()

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, May 07, 2022 at 03:19:51PM +0800, Jason Wang wrote:
> This patch tries to implement the synchronize_cbs() for ccw. For the
> vring_interrupt() that is called via virtio_airq_handler(), the
> synchronization is simply done via the airq_info's lock. For the
> vring_interrupt() that is called via virtio_ccw_int_handler(), a per
> device spinlock for irq is introduced ans used in the synchronization
> method.
> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Halil Pasic <pasic@linux.ibm.com>
> Cc: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/s390/virtio/virtio_ccw.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> index d35e7a3f7067..001e1f0e6037 100644
> --- a/drivers/s390/virtio/virtio_ccw.c
> +++ b/drivers/s390/virtio/virtio_ccw.c
> @@ -62,6 +62,7 @@ struct virtio_ccw_device {
>  	unsigned int revision; /* Transport revision */
>  	wait_queue_head_t wait_q;
>  	spinlock_t lock;
> +	rwlock_t irq_lock;
>  	struct mutex io_lock; /* Serializes I/O requests */
>  	struct list_head virtqueues;
>  	bool is_thinint;
> @@ -984,6 +985,27 @@ static const char *virtio_ccw_bus_name(struct virtio_device *vdev)
>  	return dev_name(&vcdev->cdev->dev);
>  }
>  
> +static void virtio_ccw_synchronize_cbs(struct virtio_device *vdev)
> +{
> +	struct virtio_ccw_device *vcdev = to_vc_device(vdev);
> +	struct airq_info *info = vcdev->airq_info;
> +
> +	if (info) {
> +		/*
> +		 * Synchronize with the vring_interrupt() with airq indicator
> +		 */
> +		write_lock(&info->lock);
> +		write_unlock(&info->lock);
> +	} else {
> +		/*
> +		 * Synchronize with the vring_interrupt() called by
> +		 * virtio_ccw_int_handler().
> +		 */
> +		write_lock(&vcdev->irq_lock);
> +		write_unlock(&vcdev->irq_lock);
> +	}
> +}
> +
>  static const struct virtio_config_ops virtio_ccw_config_ops = {
>  	.get_features = virtio_ccw_get_features,
>  	.finalize_features = virtio_ccw_finalize_features,
> @@ -995,6 +1017,7 @@ static const struct virtio_config_ops virtio_ccw_config_ops = {
>  	.find_vqs = virtio_ccw_find_vqs,
>  	.del_vqs = virtio_ccw_del_vqs,
>  	.bus_name = virtio_ccw_bus_name,
> +	.synchronize_cbs = virtio_ccw_synchronize_cbs,
>  };
>  
>  
> @@ -1079,6 +1102,7 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
>  {
>  	__u32 activity = intparm & VIRTIO_CCW_INTPARM_MASK;
>  	struct virtio_ccw_device *vcdev = dev_get_drvdata(&cdev->dev);
> +	unsigned long flags;
>  	int i;
>  	struct virtqueue *vq;
>  
> @@ -1106,6 +1130,7 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
>  			vcdev->err = -EIO;
>  	}
>  	virtio_ccw_check_activity(vcdev, activity);
> +	read_lock_irqsave(&vcdev->irq_lock, flags);
>  	for_each_set_bit(i, indicators(vcdev),
>  			 sizeof(*indicators(vcdev)) * BITS_PER_BYTE) {
>  		/* The bit clear must happen before the vring kick. */

Cornelia sent a lockdep trace on this.

Basically I think this gets the irqsave/restore logic wrong.
It attempts to disable irqs in the handler (which is an interrupt
anyway).
And it does not disable irqs in the synchronize_cbs.

As a result in interrupt might try to take a read lock while
.synchronize_cbs has the writer lock, resulting in a deadlock.

I think you want regular read_lock + write_lock_irq.


> @@ -1114,6 +1139,7 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
>  		vq = virtio_ccw_vq_by_ind(vcdev, i);
>  		vring_interrupt(0, vq);
>  	}
> +	read_unlock_irqrestore(&vcdev->irq_lock, flags);
>  	if (test_bit(0, indicators2(vcdev))) {
>  		virtio_config_changed(&vcdev->vdev);
>  		clear_bit(0, indicators2(vcdev));
> @@ -1284,6 +1310,7 @@ static int virtio_ccw_online(struct ccw_device *cdev)
>  	init_waitqueue_head(&vcdev->wait_q);
>  	INIT_LIST_HEAD(&vcdev->virtqueues);
>  	spin_lock_init(&vcdev->lock);
> +	rwlock_init(&vcdev->irq_lock);
>  	mutex_init(&vcdev->io_lock);
>  
>  	spin_lock_irqsave(get_ccwdev_lock(cdev), flags);
> -- 
> 2.25.1