atomic_t: add atomic64_t

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
common/perf_counters: use atomics instead of a mutex

The mutex is way too expensive.

Signed-off-by: Sage Weil <sage@inktank.com>
common/perf_counters: use second atomic to make counters safe to read

Add a second counter so that we can detect a race with an add/inc during
read, and retry.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1811 from ceph/wip-perf-atomic

perf_counter: use atomics instead of mutex

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>