From: Michael S. Tsirkin <mst@redhat.com>
Date: Wed, 28 May 2014 14:07:02 +0000 (+0300)
Subject: vhost: replace rcu with mutex
X-Git-Tag: v3.16-rc1~38^2~2
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=98f9ca0a3faa99b7388578d55eccecf272be4038;p=platform%2Fkernel%2Flinux-exynos.git

vhost: replace rcu with mutex

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>
---

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;
 }