From: Nathan Bossart <nathan@postgresql.org>
Subject: [PATCH v5 3/3] Better express platform requirements in s_lock.h.
Date: Thu, 7 May 2026 15:32:50 -0500
---
src/backend/storage/lmgr/s_lock.c | 2 ++
src/include/storage/s_lock.h | 59 +++++++++++++++++--------------
2 files changed, 35 insertions(+), 26 deletions(-)
diff --git a/src/backend/storage/lmgr/s_lock.c b/src/backend/storage/lmgr/s_lock.cindex 6df568eccb3..34c6de66773 100644--- a/src/backend/storage/lmgr/s_lock.c+++ b/src/backend/storage/lmgr/s_lock.c@@ -91,6 +91,7 @@ s_lock_stuck(const char *file, int line, const char *func)
#endif
}
+#ifdef USE_DEFAULT_S_LOCK
/*
* s_lock(lock) - platform-independent portion of waiting for a spinlock.
*/
@@ -110,6 +111,7 @@ s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
return delayStatus.delays;
}
+#endif
#ifdef USE_DEFAULT_S_UNLOCK
void
diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.hindex fb872edd2f0..65f00c06518 100644--- a/src/include/storage/s_lock.h+++ b/src/include/storage/s_lock.h@@ -44,23 +44,16 @@
* atomic test-and-set only when it appears free.
*
* TAS() and TAS_SPIN() are NOT part of the API, and should never be called
- * directly.- *- * CAUTION: on some platforms TAS() and/or TAS_SPIN() may sometimes report- * failure to acquire a lock even when the lock is not locked. For example,- * on Alpha TAS() will "fail" if interrupted. Therefore a retry loop must- * always be used, even if you are certain the lock is free.+ * directly. If a platform-specific TAS() is defined, the platform must+ * _not_ define its own S_LOCK(). Conversely, if a platform-specific+ * S_LOCK() is defined, the platform must _not_ define its own TAS().+ * Currently, all supported platforms define TAS() and use the default+ * S_LOCK() implementation, so that is probably a good place to start if+ * adding a new one.
*
* It is the responsibility of these macros to make sure that the compiler
* does not re-order accesses to shared memory to precede the actual lock
- * acquisition, or follow the lock release. Prior to PostgreSQL 9.5, this- * was the caller's responsibility, which meant that callers had to use- * volatile-qualified pointers to refer to both the spinlock itself and the- * shared data being accessed within the spinlocked critical section. This- * was notationally awkward, easy to forget (and thus error-prone), and- * prevented some useful compiler optimizations. For these reasons, we- * now require that the macros themselves prevent compiler re-ordering,- * so that the caller doesn't need to take special precautions.+ * acquisition, or follow the lock release.
*
* On platforms with weak memory ordering, the TAS(), TAS_SPIN(), and
* S_UNLOCK() macros must further include hardware-level memory fence
@@ -72,7 +65,7 @@
*
* On most supported platforms, TAS() uses a tas() function written
* in assembly language to execute a hardware atomic-test-and-set
- * instruction. Equivalent OS-supplied mutex routines could be used too.+ * instruction. Equivalent compiler intrinsics are another popular option.
*
*
* Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
@@ -642,20 +635,27 @@ spin_delay(void)
#endif /* !defined(TAS) */
-/* Blow up if we didn't have any way to do spinlocks */-#ifndef TAS-#error PostgreSQL does not have spinlock support on this platform. Please report this to pgsql-bugs@lists.postgresql.org.-#endif--
/*
* Default Definitions - override these above as needed.
*/
-#if !defined(S_LOCK)+/*+ * Make sure S_LOCK is defined, either explicitly for the platform or via a TAS+ * macro for the platform. Exactly one of either S_LOCK or TAS should be+ * defined for a supported platform at this point in the file.+ */+#if defined(S_LOCK)+#if defined(TAS)+#error Both TAS and S_LOCK defined on this platform. Please report this to pgsql-bugs@lists.postgresql.org.+#endif+#elif defined(TAS)+#define USE_DEFAULT_S_LOCK+extern int s_lock(volatile slock_t *lock, const char *file, int line, const char *func);
#define S_LOCK(lock) \
(TAS(lock) ? s_lock((lock), __FILE__, __LINE__, __func__) : 0)
-#endif /* S_LOCK */+#else+#error Neither TAS nor S_LOCK defined on this platform. Please report this to pgsql-bugs@lists.postgresql.org.+#endif
#if !defined(S_UNLOCK)
/*
@@ -687,15 +687,22 @@ extern void s_unlock(volatile slock_t *lock);
#define SPIN_DELAY() ((void) 0)
#endif /* SPIN_DELAY */
-#if !defined(TAS_SPIN)+/*+ * We can only define TAS_SPIN if TAS was defined. Otherwise, the platform+ * defined its own S_LOCK without TAS. Since TAS_SPIN is only used by the+ * default S_LOCK's helper function, there's no need to define TAS_SPIN at all+ * in that case, unless you plan to use it in a platform-specific S_LOCK helper+ * function. (Note that we currently do not have any platforms that don't+ * define TAS.)+ */+#if !defined(TAS_SPIN) && defined(TAS)
#define TAS_SPIN(lock) TAS(lock)
-#endif /* TAS_SPIN */+#endif /* ! TAS_SPIN && TAS */
/*
* Platform-independent out-of-line support routines
*/
-extern int s_lock(volatile slock_t *lock, const char *file, int line, const char *func);
/* Support for dynamic adjustment of spins_per_delay */
#define DEFAULT_SPINS_PER_DELAY 100
--
2.50.1 (Apple Git-155)
--EzTP03ol5OjsRjZr--
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To: pgsql-hackers@postgresql.org
Cc: nathan@postgresql.org
Subject: Re: [PATCH v5 3/3] Better express platform requirements in s_lock.h.
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