On Wed, 2012-06-06 at 19:57 +0300, Michael S. Tsirkin wrote:

So for virtio since all counters get incremented from bh we can
ensure they are read atomically, simply but reading them
from the correct CPU with bh disabled.
And then we don't need u64_stats_sync at all.

Really ? How are you going to read 64bit stats from foreign cpus on
32bit arches, without additional cost in fast path ?

You should read include/linux/u64_stats_sync.h to fully understand the
issues.