On 2019/8/7 下午8:07, Jason Gunthorpe wrote:
On Wed, Aug 07, 2019 at 03:06:15AM -0400, Jason Wang wrote:
We used to use RCU to synchronize MMU notifier with worker. This leads
calling synchronize_rcu() in invalidate_range_start(). But on a busy
system, there would be many factors that may slow down the
synchronize_rcu() which makes it unsuitable to be called in MMU
notifier.
So this patch switches use seqlock counter to track whether or not the
map was used. The counter was increased when vq try to start or finish
uses the map. This means, when it was even, we're sure there's no
readers and MMU notifier is synchronized. When it was odd, it means
there's a reader we need to wait it to be even again then we are
synchronized. Consider the read critical section is pretty small the
synchronization should be done very fast.
Reported-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
drivers/vhost/vhost.c | 141 ++++++++++++++++++++++++++----------------
drivers/vhost/vhost.h | 7 ++-
2 files changed, 90 insertions(+), 58 deletions(-)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index cfc11f9ed9c9..57bfbb60d960 100644
+++ b/drivers/vhost/vhost.c
@@ -324,17 +324,16 @@ static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq)
spin_lock(&vq->mmu_lock);
for (i = 0; i < VHOST_NUM_ADDRS; i++) {
- map[i] = rcu_dereference_protected(vq->maps[i],
- lockdep_is_held(&vq->mmu_lock));
+ map[i] = vq->maps[i];
if (map[i]) {
vhost_set_map_dirty(vq, map[i], i);
- rcu_assign_pointer(vq->maps[i], NULL);
+ vq->maps[i] = NULL;
}
}
spin_unlock(&vq->mmu_lock);
- /* No need for synchronize_rcu() or kfree_rcu() since we are
- * serialized with memory accessors (e.g vq mutex held).
+ /* No need for synchronization since we are serialized with
+ * memory accessors (e.g vq mutex held).
*/
for (i = 0; i < VHOST_NUM_ADDRS; i++)
@@ -362,6 +361,40 @@ static bool vhost_map_range_overlap(struct vhost_uaddr *uaddr,
return !(end < uaddr->uaddr || start > uaddr->uaddr - 1 + uaddr->size);
}
+static void inline vhost_vq_access_map_begin(struct vhost_virtqueue *vq)
+{
+ write_seqcount_begin(&vq->seq);
+}
+
+static void inline vhost_vq_access_map_end(struct vhost_virtqueue *vq)
+{
+ write_seqcount_end(&vq->seq);
+}
The write side of a seqlock only provides write barriers. Access to
map = vq->maps[VHOST_ADDR_USED];
Still needs a read side barrier, and then I think this will be no
better than a normal spinlock.
It also doesn't seem like this algorithm even needs a seqlock, as this
is just a one bit flag
Right, so then I tend to use spinlock first for correctness.
atomic_set_bit(using map)
smp_mb__after_atomic()
.. maps [...]
atomic_clear_bit(using map)
map = NULL;
smp_mb__before_atomic();
while (atomic_read_bit(using map))
relax()
Again, not clear this could be faster than a spinlock when the
barriers are correct...
Yes, for next release we may want to use the idea from Michael like to
mitigate the impact of mb.
https://lwn.net/Articles/775871/
Thanks
Jason
