Remove --disable-spinlocks.

A later change will require atomic support, so it wouldn't make sense
for a hypothetical new system not to be able to implement spinlocks.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (concept, not the patch)
Reviewed-by: Andres Freund <andres@anarazel.de> (concept, not the patch)
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us

6 files changed, 9 insertions, 74 deletions

diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 3dea3856aaf..e6c06f61027 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -382,9 +382,6 @@
 /* Define to 1 if the system has the type `socklen_t'. */
 #undef HAVE_SOCKLEN_T
 
-/* Define to 1 if you have spinlocks. */
-#undef HAVE_SPINLOCKS
-
 /* Define to 1 if you have the `SSL_CTX_set_cert_cb' function. */
 #undef HAVE_SSL_CTX_SET_CERT_CB
 
diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index f941ee2faf8..11f74f4b56d 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -86,21 +86,6 @@
 #define USE_FLOAT8_BYVAL 1
 #endif
 
-/*
- * When we don't have native spinlocks, we use semaphores to simulate them.
- * Decreasing this value reduces consumption of OS resources; increasing it
- * may improve performance, but supplying a real spinlock implementation is
- * probably far better.
- */
-#define NUM_SPINLOCK_SEMAPHORES		128
-
-/*
- * When we have neither spinlocks nor atomic operations support we're
- * implementing atomic operations on top of spinlock on top of semaphores. To
- * be safe against atomic operations while holding a spinlock separate
- * semaphores have to be used.
- */
-#define NUM_ATOMICS_SEMAPHORES		64
 
 /*
  * MAXPGPATH: standard size of a pathname buffer in PostgreSQL (hence,
diff --git a/src/include/port/atomics.h b/src/include/port/atomics.h
index f6fa432d2df..03134e3b7bb 100644
--- a/src/include/port/atomics.h
+++ b/src/include/port/atomics.h
@@ -16,8 +16,8 @@
  *
  * There exist generic, hardware independent, implementations for several
  * compilers which might be sufficient, although possibly not optimal, for a
- * new platform. If no such generic implementation is available spinlocks (or
- * even OS provided semaphores) will be used to implement the API.
+ * new platform. If no such generic implementation is available spinlocks will
+ * be used to implement the API.
  *
  * Implement _u64 atomics if and only if your platform can use them
  * efficiently (and obviously correctly).
diff --git a/src/include/port/atomics/fallback.h b/src/include/port/atomics/fallback.h
index 34cfee110fb..2e3eef4acaf 100644
--- a/src/include/port/atomics/fallback.h
+++ b/src/include/port/atomics/fallback.h
@@ -20,9 +20,7 @@
 #ifndef pg_memory_barrier_impl
 /*
  * If we have no memory barrier implementation for this architecture, we
- * fall back to acquiring and releasing a spinlock.  This might, in turn,
- * fall back to the semaphore-based spinlock implementation, which will be
- * amazingly slow.
+ * fall back to acquiring and releasing a spinlock.
  *
  * It's not self-evident that every possible legal implementation of a
  * spinlock acquire-and-release would be equivalent to a full memory barrier.
diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index 02c68513a53..e94ed5f48bd 100644
--- a/src/include/storage/s_lock.h
+++ b/src/include/storage/s_lock.h
@@ -1,10 +1,10 @@
 /*-------------------------------------------------------------------------
  *
  * s_lock.h
- *	   Hardware-dependent implementation of spinlocks.
+ *	   Implementation of spinlocks.
  *
  *	NOTE: none of the macros in this file are intended to be called directly.
- *	Call them through the hardware-independent macros in spin.h.
+ *	Call them through the macros in spin.h.
  *
  *	The following hardware-dependent macros must be provided for each
  *	supported platform:
@@ -78,13 +78,6 @@
  *	in assembly language to execute a hardware atomic-test-and-set
  *	instruction.  Equivalent OS-supplied mutex routines could be used too.
  *
- *	If no system-specific TAS() is available (ie, HAVE_SPINLOCKS is not
- *	defined), then we fall back on an emulation that uses SysV semaphores
- *	(see spin.c).  This emulation will be MUCH MUCH slower than a proper TAS()
- *	implementation, because of the cost of a kernel call per lock or unlock.
- *	An old report is that Postgres spends around 40% of its time in semop(2)
- *	when using the SysV semaphore code.
- *
  *
  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -100,8 +93,6 @@
 #error "s_lock.h may not be included from frontend code"
 #endif
 
-#ifdef HAVE_SPINLOCKS	/* skip spinlocks if requested */
-
 #if defined(__GNUC__) || defined(__INTEL_COMPILER)
 /*************************************************************************
  * All the gcc inlines
@@ -655,34 +646,10 @@ spin_delay(void)
 
 /* Blow up if we didn't have any way to do spinlocks */
 #ifndef HAS_TEST_AND_SET
-#error PostgreSQL does not have native spinlock support on this platform.  To continue the compilation, rerun configure using --disable-spinlocks.  However, performance will be poor.  Please report this to pgsql-bugs@lists.postgresql.org.
+#error PostgreSQL does not have spinlock support on this platform.  Please report this to pgsql-bugs@lists.postgresql.org.
 #endif
 
 
-#else	/* !HAVE_SPINLOCKS */
-
-
-/*
- * Fake spinlock implementation using semaphores --- slow and prone
- * to fall foul of kernel limits on number of semaphores, so don't use this
- * unless you must!  The subroutines appear in spin.c.
- */
-typedef int slock_t;
-
-extern bool s_lock_free_sema(volatile slock_t *lock);
-extern void s_unlock_sema(volatile slock_t *lock);
-extern void s_init_lock_sema(volatile slock_t *lock, bool nested);
-extern int	tas_sema(volatile slock_t *lock);
-
-#define S_LOCK_FREE(lock)	s_lock_free_sema(lock)
-#define S_UNLOCK(lock)	 s_unlock_sema(lock)
-#define S_INIT_LOCK(lock)	s_init_lock_sema(lock, false)
-#define TAS(lock)	tas_sema(lock)
-
-
-#endif	/* HAVE_SPINLOCKS */
-
-
 /*
  * Default Definitions - override these above as needed.
  */
diff --git a/src/include/storage/spin.h b/src/include/storage/spin.h
index c0679c59992..3ae2a56d073 100644
--- a/src/include/storage/spin.h
+++ b/src/include/storage/spin.h
@@ -1,11 +1,11 @@
 /*-------------------------------------------------------------------------
  *
  * spin.h
- *	   Hardware-independent implementation of spinlocks.
+ *	   API for spinlocks.
  *
  *
- *	The hardware-independent interface to spinlocks is defined by the
- *	typedef "slock_t" and these macros:
+ *	The interface to spinlocks is defined by the typedef "slock_t" and
+ *	these macros:
  *
  *	void SpinLockInit(volatile slock_t *lock)
  *		Initialize a spinlock (to the unlocked state).
@@ -52,9 +52,6 @@
 #define SPIN_H
 
 #include "storage/s_lock.h"
-#ifndef HAVE_SPINLOCKS
-#include "storage/pg_sema.h"
-#endif
 
 
 #define SpinLockInit(lock)	S_INIT_LOCK(lock)
@@ -65,13 +62,4 @@
 
 #define SpinLockFree(lock)	S_LOCK_FREE(lock)
 
-
-extern int	SpinlockSemas(void);
-extern Size SpinlockSemaSize(void);
-
-#ifndef HAVE_SPINLOCKS
-extern void SpinlockSemaInit(void);
-extern PGDLLIMPORT PGSemaphore *SpinlockSemaArray;
-#endif
-
 #endif							/* SPIN_H */