Re: [RFC PATCH v6 17/20] vfio: add local lock in virqfd instead of depending on VFIO PCI

[Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Thu, 2014-06-05 at 19:03 +0200, Antonios Motakis wrote:
> Sharing the same spinlock with the VFIO bus driver is not necessary for
> the virqfd code, so remove that dependency.

I like the idea of consolidating this code, but I need more
justification for why the use of the lock here is independent of the bus
driver and why it's ok to move from a per-device lock to a global lock.
Thanks,

Alex

> 
> Signed-off-by: Antonios Motakis <a.motakis-lrHrjnjw1UfHK3s98zE1ajGjJy/sRE9J@public.gmane.org>
> ---
>  drivers/vfio/pci/vfio_pci_intrs.c | 10 +++++-----
>  drivers/vfio/virqfd.c             | 24 +++++++++++++-----------
>  include/linux/vfio.h              |  3 +--
>  3 files changed, 19 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> index 3f909bb..e56c814 100644
> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> @@ -226,8 +226,8 @@ static int vfio_intx_set_signal(struct vfio_pci_device *vdev, int fd)
>  static void vfio_intx_disable(struct vfio_pci_device *vdev)
>  {
>  	vfio_intx_set_signal(vdev, -1);
> -	virqfd_disable(vdev, &vdev->ctx[0].unmask);
> -	virqfd_disable(vdev, &vdev->ctx[0].mask);
> +	virqfd_disable(&vdev->ctx[0].unmask);
> +	virqfd_disable(&vdev->ctx[0].mask);
>  	vdev->irq_type = VFIO_PCI_NUM_IRQS;
>  	vdev->num_ctx = 0;
>  	kfree(vdev->ctx);
> @@ -377,8 +377,8 @@ static void vfio_msi_disable(struct vfio_pci_device *vdev, bool msix)
>  	vfio_msi_set_block(vdev, 0, vdev->num_ctx, NULL, msix);
>  
>  	for (i = 0; i < vdev->num_ctx; i++) {
> -		virqfd_disable(vdev, &vdev->ctx[i].unmask);
> -		virqfd_disable(vdev, &vdev->ctx[i].mask);
> +		virqfd_disable(&vdev->ctx[i].unmask);
> +		virqfd_disable(&vdev->ctx[i].mask);
>  	}
>  
>  	if (msix) {
> @@ -415,7 +415,7 @@ static int vfio_pci_set_intx_unmask(struct vfio_pci_device *vdev,
>  					     vfio_send_intx_eventfd, NULL,
>  					     &vdev->ctx[0].unmask, fd);
>  
> -		virqfd_disable(vdev, &vdev->ctx[0].unmask);
> +		virqfd_disable(&vdev->ctx[0].unmask);
>  	}
>  
>  	return 0;
> diff --git a/drivers/vfio/virqfd.c b/drivers/vfio/virqfd.c
> index 9cf2842..4528450 100644
> --- a/drivers/vfio/virqfd.c
> +++ b/drivers/vfio/virqfd.c
> @@ -17,6 +17,7 @@
>  #include "pci/vfio_pci_private.h"
>  
>  static struct workqueue_struct *vfio_irqfd_cleanup_wq;
> +static spinlock_t lock;
>  
>  int __init vfio_pci_virqfd_init(void)
>  {
> @@ -25,6 +26,8 @@ int __init vfio_pci_virqfd_init(void)
>  	if (!vfio_irqfd_cleanup_wq)
>  		return -ENOMEM;
>  
> +	spin_lock_init(&lock);
> +
>  	return 0;
>  }
>  
> @@ -53,21 +56,21 @@ static int virqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
>  
>  	if (flags & POLLHUP) {
>  		unsigned long flags;
> -		spin_lock_irqsave(&virqfd->vdev->irqlock, flags);
> +		spin_lock_irqsave(&lock, flags);
>  
>  		/*
>  		 * The eventfd is closing, if the virqfd has not yet been
>  		 * queued for release, as determined by testing whether the
> -		 * vdev pointer to it is still valid, queue it now.  As
> +		 * virqfd pointer to it is still valid, queue it now.  As
>  		 * with kvm irqfds, we know we won't race against the virqfd
> -		 * going away because we hold wqh->lock to get here.
> +		 * going away because we hold the lock to get here.
>  		 */
>  		if (*(virqfd->pvirqfd) == virqfd) {
>  			*(virqfd->pvirqfd) = NULL;
>  			virqfd_deactivate(virqfd);
>  		}
>  
> -		spin_unlock_irqrestore(&virqfd->vdev->irqlock, flags);
> +		spin_unlock_irqrestore(&lock, flags);
>  	}
>  
>  	return 0;
> @@ -143,16 +146,16 @@ int virqfd_enable(struct vfio_pci_device *vdev,
>  	 * we update the pointer to the virqfd under lock to avoid
>  	 * pushing multiple jobs to release the same virqfd.
>  	 */
> -	spin_lock_irq(&vdev->irqlock);
> +	spin_lock_irq(&lock);
>  
>  	if (*pvirqfd) {
> -		spin_unlock_irq(&vdev->irqlock);
> +		spin_unlock_irq(&lock);
>  		ret = -EBUSY;
>  		goto err_busy;
>  	}
>  	*pvirqfd = virqfd;
>  
> -	spin_unlock_irq(&vdev->irqlock);
> +	spin_unlock_irq(&lock);
>  
>  	/*
>  	 * Install our own custom wake-up handling so we are notified via
> @@ -189,19 +192,18 @@ err_fd:
>  	return ret;
>  }
>  
> -void virqfd_disable(struct vfio_pci_device *vdev,
> -			   struct virqfd **pvirqfd)
> +void virqfd_disable(struct virqfd **pvirqfd)
>  {
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&vdev->irqlock, flags);
> +	spin_lock_irqsave(&lock, flags);
>  
>  	if (*pvirqfd) {
>  		virqfd_deactivate(*pvirqfd);
>  		*pvirqfd = NULL;
>  	}
>  
> -	spin_unlock_irqrestore(&vdev->irqlock, flags);
> +	spin_unlock_irqrestore(&lock, flags);
>  
>  	/*
>  	 * Block until we know all outstanding shutdown jobs have completed.
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index 43968e8..44c9808 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -123,7 +123,6 @@ extern int virqfd_enable(struct vfio_pci_device *vdev,
>  			 int (*handler)(struct vfio_pci_device *, void *),
>  			 void (*thread)(struct vfio_pci_device *, void *),
>  			 void *data, struct virqfd **pvirqfd, int fd);
> -extern void virqfd_disable(struct vfio_pci_device *vdev,
> -			   struct virqfd **pvirqfd);
> +extern void virqfd_disable(struct virqfd **pvirqfd);
>  
>  #endif /* VFIO_H */