Re: [PATCH RFC v2 02/10] vringh: add 'iotlb_lock' to synchronize iotlb accesses

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On Fri, Jan 29, 2021 at 03:43:40PM +0800, Jason Wang wrote:

On 2021/1/28 下午10:41, Stefano Garzarella wrote:
Usually iotlb accesses are synchronized with a spinlock.
Let's request it as a new parameter in vringh_set_iotlb() and
hold it when we navigate the iotlb in iotlb_translate() to avoid
race conditions with any new additions/deletions of ranges from
the ioltb.


Patch looks fine but I wonder if this is the best approach comparing to do locking by the caller.

Initially I tried to hold the lock in the vdpasim_blk_work(), but since
we have a lot of different functions for vringh, I opted to take the lock at the beginning and release it at the end. Also because several times I went to see if that call used iotlb_translate or not.

This could be a problem for example if we have multiple workers to handle multiple queues.

Also, some functions are quite long (e.g. vringh_getdesc_iotlb) and holding the lock for that long could reduce parallelism.

For these reasons I thought it was better to hide everything from the caller who doesn't have to worry about which function calls iotlb_translate() and thus hold the lock.

Thanks,
Stefano


Thanks



Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
---
 include/linux/vringh.h           | 6 +++++-
 drivers/vdpa/vdpa_sim/vdpa_sim.c | 3 ++-
 drivers/vhost/vringh.c           | 9 ++++++++-
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/include/linux/vringh.h b/include/linux/vringh.h
index 59bd50f99291..9c077863c8f6 100644
--- a/include/linux/vringh.h
+++ b/include/linux/vringh.h
@@ -46,6 +46,9 @@ struct vringh {
 	/* IOTLB for this vring */
 	struct vhost_iotlb *iotlb;
+	/* spinlock to synchronize IOTLB accesses */
+	spinlock_t *iotlb_lock;
+
 	/* The function to call to notify the guest about added buffers */
 	void (*notify)(struct vringh *);
 };
@@ -258,7 +261,8 @@ static inline __virtio64 cpu_to_vringh64(const struct vringh *vrh, u64 val)
 #if IS_REACHABLE(CONFIG_VHOST_IOTLB)
-void vringh_set_iotlb(struct vringh *vrh, struct vhost_iotlb *iotlb);
+void vringh_set_iotlb(struct vringh *vrh, struct vhost_iotlb *iotlb,
+		      spinlock_t *iotlb_lock);
 int vringh_init_iotlb(struct vringh *vrh, u64 features,
 		      unsigned int num, bool weak_barriers,
diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
index 2183a833fcf4..53238989713d 100644
--- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
+++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
@@ -284,7 +284,8 @@ struct vdpasim *vdpasim_create(struct vdpasim_dev_attr *dev_attr)
 		goto err_iommu;
 	for (i = 0; i < dev_attr->nvqs; i++)
-		vringh_set_iotlb(&vdpasim->vqs[i].vring, vdpasim->iommu);
+		vringh_set_iotlb(&vdpasim->vqs[i].vring, vdpasim->iommu,
+				 &vdpasim->iommu_lock);
 	ret = iova_cache_get();
 	if (ret)
diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index 85d85faba058..f68122705719 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -1074,6 +1074,8 @@ static int iotlb_translate(const struct vringh *vrh,
 	int ret = 0;
 	u64 s = 0;
+	spin_lock(vrh->iotlb_lock);
+
 	while (len > s) {
 		u64 size, pa, pfn;
@@ -1103,6 +1105,8 @@ static int iotlb_translate(const struct vringh *vrh,
 		++ret;
 	}
+	spin_unlock(vrh->iotlb_lock);
+
 	return ret;
 }
@@ -1262,10 +1266,13 @@ EXPORT_SYMBOL(vringh_init_iotlb);
  * vringh_set_iotlb - initialize a vringh for a ring with IOTLB.
  * @vrh: the vring
  * @iotlb: iotlb associated with this vring
+ * @iotlb_lock: spinlock to synchronize the iotlb accesses
  */
-void vringh_set_iotlb(struct vringh *vrh, struct vhost_iotlb *iotlb)
+void vringh_set_iotlb(struct vringh *vrh, struct vhost_iotlb *iotlb,
+		      spinlock_t *iotlb_lock)
 {
 	vrh->iotlb = iotlb;
+	vrh->iotlb_lock = iotlb_lock;
 }
 EXPORT_SYMBOL(vringh_set_iotlb);


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