On Mon, Jun 02, 2014 at 02:58:00PM -0700, Eric Dumazet wrote:

On Tue, 2014-06-03 at 00:30 +0300, Michael S. Tsirkin wrote:

quoted

All memory accesses are done under some VQ mutex.
So lock/unlock all VQs is a faster equivalent of synchronize_rcu()
for memory access changes.
Some guests cause a lot of these changes, so it's helpful
to make them faster.

Reported-by: "Gonglei (Arei)" <arei.gonglei@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/vhost/vhost.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 78987e4..1c05e60 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -593,6 +593,7 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 {
 	struct vhost_memory mem, *newmem, *oldmem;
 	unsigned long size = offsetof(struct vhost_memory, regions);
+	int i;
 
 	if (copy_from_user(&mem, m, size))
 		return -EFAULT;

@@ -619,7 +620,14 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 	oldmem = rcu_dereference_protected(d->memory,
 					   lockdep_is_held(&d->mutex));
 	rcu_assign_pointer(d->memory, newmem);
-	synchronize_rcu();
+
+	/* All memory accesses are done under some VQ mutex.
+	 * So below is a faster equivalent of synchronize_rcu()
+	 */
+	for (i = 0; i < d->nvqs; ++i) {
+		mutex_lock(&d->vqs[i]->mutex);
+		mutex_unlock(&d->vqs[i]->mutex);
+	}
 	kfree(oldmem);
 	return 0;
 }

This looks dubious

What about using kfree_rcu() instead ?

Unfortunately userspace relies on the fact that no one
uses the old mappings by the time ioctl returns.
The issue isn't freeing the memory.

translate_desc() still uses rcu_read_lock(), its not clear if the mutex
is really held.

Thanks, good point, we can drop that rcu_read_lock now, but I think this could be a
patch on top.

-- 
MST