datpath: Avoid reporting half updated statistics.

datpath: Avoid reporting half updated statistics.

We enforce mutual exclusion when updating statistics by disabling
bottom halves and only writing to per-CPU state.  However, reading
requires looking at the statistics for foreign CPUs, which could be
in the process of updating them since there isn't a lock.  This means
we could get garbage values for 64-bit values on 32-bit machines or
byte counts that don't correspond to packet counts, etc.

This commit introduces a sequence lock for statistics values to avoid
this problem.  Getting a write lock is very cheap - it only requires
incrementing a counter plus a memory barrier (which is compiled away
on x86) to acquire or release the lock and will never block.  On
read we spin until the sequence number hasn't changed in the middle
of the operation, indicating that the we have a consistent set of
values.

Signed-off-by: Jesse Gross <jesse@nicira.com>