vfio/pci: Implement ioeventfd thread handler for contended memory lock

The ioeventfd is called under spinlock with interrupts disabled,
therefore if the memory lock is contended defer code that might
sleep to a thread context.

Fixes: bc93b9a ("vfio-pci: Avoid recursive read-lock usage")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=209253#c1
Reported-by: Ian Pilcher <arequipeno@gmail.com>
Tested-by: Ian Pilcher <arequipeno@gmail.com>
Tested-by: Justin Gatzen <justin.gatzen@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: Alex Williamson

drivers/vfio/pci/vfio_pci_rdwr.c

@@ -356,34 +356,60 @@ ssize_t vfio_pci_vga_rw(struct vfio_pci_device *vdev, char __user *buf,
 	return done;
 }
 
-static int vfio_pci_ioeventfd_handler(void *opaque, void *unused)
+static void vfio_pci_ioeventfd_do_write(struct vfio_pci_ioeventfd *ioeventfd,
+					bool test_mem)
 {
-	struct vfio_pci_ioeventfd *ioeventfd = opaque;
-
 	switch (ioeventfd->count) {
 	case 1:
-		vfio_pci_iowrite8(ioeventfd->vdev, ioeventfd->test_mem,
+		vfio_pci_iowrite8(ioeventfd->vdev, test_mem,
 				  ioeventfd->data, ioeventfd->addr);
 		break;
 	case 2:
-		vfio_pci_iowrite16(ioeventfd->vdev, ioeventfd->test_mem,
+		vfio_pci_iowrite16(ioeventfd->vdev, test_mem,
 				   ioeventfd->data, ioeventfd->addr);
 		break;
 	case 4:
-		vfio_pci_iowrite32(ioeventfd->vdev, ioeventfd->test_mem,
+		vfio_pci_iowrite32(ioeventfd->vdev, test_mem,
 				   ioeventfd->data, ioeventfd->addr);
 		break;
 #ifdef iowrite64
 	case 8:
-		vfio_pci_iowrite64(ioeventfd->vdev, ioeventfd->test_mem,
+		vfio_pci_iowrite64(ioeventfd->vdev, test_mem,
 				   ioeventfd->data, ioeventfd->addr);
 		break;
 #endif
 	}
+}
+
+static int vfio_pci_ioeventfd_handler(void *opaque, void *unused)
+{
+	struct vfio_pci_ioeventfd *ioeventfd = opaque;
+	struct vfio_pci_device *vdev = ioeventfd->vdev;
+
+	if (ioeventfd->test_mem) {
+		if (!down_read_trylock(&vdev->memory_lock))
+			return 1; /* Lock contended, use thread */
+		if (!__vfio_pci_memory_enabled(vdev)) {
+			up_read(&vdev->memory_lock);
+			return 0;
+		}
+	}
+
+	vfio_pci_ioeventfd_do_write(ioeventfd, false);
+
+	if (ioeventfd->test_mem)
+		up_read(&vdev->memory_lock);
 
 	return 0;
 }
 
+static void vfio_pci_ioeventfd_thread(void *opaque, void *unused)
+{
+	struct vfio_pci_ioeventfd *ioeventfd = opaque;
+
+	vfio_pci_ioeventfd_do_write(ioeventfd, ioeventfd->test_mem);
+}
+
 long vfio_pci_ioeventfd(struct vfio_pci_device *vdev, loff_t offset,
 			uint64_t data, int count, int fd)
 {
@@ -457,7 +483,8 @@ long vfio_pci_ioeventfd(struct vfio_pci_device *vdev, loff_t offset,
 	ioeventfd->test_mem = vdev->pdev->resource[bar].flags & IORESOURCE_MEM;
 
 	ret = vfio_virqfd_enable(ioeventfd, vfio_pci_ioeventfd_handler,
-				 NULL, NULL, &ioeventfd->virqfd, fd);
+				 vfio_pci_ioeventfd_thread, NULL,
+				 &ioeventfd->virqfd, fd);
 	if (ret) {
 		kfree(ioeventfd);
 		goto out_unlock;