pgsql: Avoid acquiring spinlock when checking if recovery has finished,

Avoid acquiring spinlock when checking if recovery has finished, for speed.

RecoveryIsInProgress() can be called very frequently. During normal
operation, it just checks a backend-local variable and returns quickly,
but during hot standby, it checks a spinlock-protected shared variable.
Those spinlock acquisitions can become a point of contention on a busy
hot standby system.

Replace the spinlock acquisition with a memory barrier.

Per discussion with Andres Freund, Ants Aasma and Merlin Moncure.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/1a3d104475ce01326fc00601ed66ac4d658e37e5

Modified Files
--------------
src/backend/access/transam/xlog.c |   21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)