[PATCH v3 6/6] virtio_vdpa: implement synchronize_cbs()

[Karl Mehltretter <kmehltretter@gmail.com>]

virtio_vdpa relies on the core's synchronize_rcu() fallback for callback
synchronization. That covers IRQ handlers and sections with interrupts,
preemption or bottom halves disabled. virtio_vdpa does not enforce any
of those contexts when a vDPA driver invokes a callback.

Add an SRCU domain per device and enter it around the virtqueue and
config callbacks. Implement synchronize_cbs() with synchronize_srcu()
so it covers the callbacks regardless of the calling context.

SRCU uses per-CPU reader accounting, avoiding a single callback lock
shared by all queues. Cover the config callback too, as virtio-pci
does for its config vector.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 drivers/virtio/virtio_vdpa.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
index 6bcf4567a929..f9559b6b2e0b 100644
--- a/drivers/virtio/virtio_vdpa.c
+++ b/drivers/virtio/virtio_vdpa.c
@@ -12,6 +12,7 @@
 #include <linux/device.h>
 #include <linux/kernel.h>
 #include <linux/slab.h>
+#include <linux/srcu.h>
 #include <linux/uuid.h>
 #include <linux/group_cpus.h>
 #include <linux/virtio.h>
@@ -27,6 +28,7 @@
 struct virtio_vdpa_device {
 	struct virtio_device vdev;
 	struct vdpa_device *vdpa;
+	struct srcu_struct cb_srcu;
 	u64 features;
 };
 
@@ -114,8 +116,11 @@ static bool virtio_vdpa_notify_with_data(struct virtqueue *vq)
 static irqreturn_t virtio_vdpa_config_cb(void *private)
 {
 	struct virtio_vdpa_device *vd_dev = private;
+	int idx;
 
+	idx = srcu_read_lock(&vd_dev->cb_srcu);
 	virtio_config_changed(&vd_dev->vdev);
+	srcu_read_unlock(&vd_dev->cb_srcu, idx);
 
 	return IRQ_HANDLED;
 }
@@ -123,8 +128,22 @@ static irqreturn_t virtio_vdpa_config_cb(void *private)
 static irqreturn_t virtio_vdpa_virtqueue_cb(void *private)
 {
 	struct virtqueue *vq = private;
+	struct virtio_vdpa_device *vd_dev = to_virtio_vdpa_device(vq->vdev);
+	irqreturn_t ret;
+	int idx;
+
+	idx = srcu_read_lock(&vd_dev->cb_srcu);
+	ret = vring_interrupt(0, vq);
+	srcu_read_unlock(&vd_dev->cb_srcu, idx);
+
+	return ret;
+}
+
+static void virtio_vdpa_synchronize_cbs(struct virtio_device *vdev)
+{
+	struct virtio_vdpa_device *vd_dev = to_virtio_vdpa_device(vdev);
 
-	return vring_interrupt(0, vq);
+	synchronize_srcu(&vd_dev->cb_srcu);
 }
 
 static struct virtqueue *
@@ -440,6 +459,7 @@ static const struct virtio_config_ops virtio_vdpa_config_ops = {
 	.reset		= virtio_vdpa_reset,
 	.find_vqs	= virtio_vdpa_find_vqs,
 	.del_vqs	= virtio_vdpa_del_vqs,
+	.synchronize_cbs = virtio_vdpa_synchronize_cbs,
 	.get_features	= virtio_vdpa_get_features,
 	.finalize_features = virtio_vdpa_finalize_features,
 	.bus_name	= virtio_vdpa_bus_name,
@@ -454,6 +474,7 @@ static void virtio_vdpa_release_dev(struct device *_d)
 	struct virtio_vdpa_device *vd_dev =
 	       container_of(vdev, struct virtio_vdpa_device, vdev);
 
+	cleanup_srcu_struct(&vd_dev->cb_srcu);
 	kfree(vd_dev);
 }
 
@@ -467,6 +488,11 @@ static int virtio_vdpa_probe(struct vdpa_device *vdpa)
 	if (!vd_dev)
 		return -ENOMEM;
 
+	if (init_srcu_struct(&vd_dev->cb_srcu)) {
+		kfree(vd_dev);
+		return -ENOMEM;
+	}
+
 	vd_dev->vdev.dev.parent = vdpa->map ? &vdpa->dev :
 				  vdpa_get_map(vdpa).dma_dev;
 	vd_dev->vdev.dev.release = virtio_vdpa_release_dev;
@@ -489,10 +515,12 @@ static int virtio_vdpa_probe(struct vdpa_device *vdpa)
 	return 0;
 
 err:
-	if (reg_dev)
+	if (reg_dev) {
 		put_device(&vd_dev->vdev.dev);
-	else
+	} else {
+		cleanup_srcu_struct(&vd_dev->cb_srcu);
 		kfree(vd_dev);
+	}
 	return ret;
 }
 
-- 
2.39.5 (Apple Git-154)