spinlock-delay-first.patch

From bf72499bfecd4a6ea4757a445a2b711461639a56 Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@enterprisedb.com>
Date: Fri, 15 Jun 2018 16:17:51 +1200
Subject: [PATCH] Experimental spinlock changes on amd64.  Not tested.

Based on code review from mjg@freebsd.org, the LOCK prefix is redundant and
serves only to make the .text segment larger, and the fallback behaviour after
TAS() returns true should be to delay first, and then TAS_SPIN().

It may be that the logic in finish_spin_delay needs tweaking.
---
 src/backend/storage/lmgr/s_lock.c | 4 ++--
 src/include/storage/s_lock.h      | 1 -
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/src/backend/storage/lmgr/s_lock.c b/src/backend/storage/lmgr/s_lock.c
index 7482b2d44b..84de03b0fc 100644
--- a/src/backend/storage/lmgr/s_lock.c
+++ b/src/backend/storage/lmgr/s_lock.c
@@ -95,10 +95,10 @@ s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
 
        init_spin_delay(&delayStatus, file, line, func);
 
-       while (TAS_SPIN(lock))
+       do
        {
                perform_spin_delay(&delayStatus);
-       }
+       } while (TAS_SPIN(lock));
 
        finish_spin_delay(&delayStatus);
 
diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index 04175dbaaa..aede2c7ddc 100644
--- a/src/include/storage/s_lock.h
+++ b/src/include/storage/s_lock.h
@@ -226,7 +226,6 @@ tas(volatile slock_t *lock)
        register slock_t _res = 1;
 
        __asm__ __volatile__(
-               "       lock                    \n"
                "       xchgb   %0,%1   \n"
 :              "+q"(_res), "+m"(*lock)
 :              /* no inputs */
-- 
2.17.0