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help / color / mirror / Atom feedFrom: Nathan Bossart <[email protected]>
Subject: [PATCH v4 3/3] Better express platform requirements in s_lock.h.
Date: Thu, 7 May 2026 15:32:50 -0500
---
src/include/storage/s_lock.h | 56 ++++++++++++++++++++----------------
1 file changed, 32 insertions(+), 24 deletions(-)
diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index fb872edd2f0..c4369ee4ff6 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(), but
+ * it does need to define its own TAS_SPIN(). 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 +635,25 @@ 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 [email protected].
-#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 [email protected].
+#endif
+#elif defined(TAS)
#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 [email protected].
+#endif
#if !defined(S_UNLOCK)
/*
@@ -687,9 +685,19 @@ extern void s_unlock(volatile slock_t *lock);
#define SPIN_DELAY() ((void) 0)
#endif /* SPIN_DELAY */
+/*
+ * We can only define TAS_SPIN if TAS was defined. Otherwise, the platform
+ * defined its own S_LOCK without TAS, and therefore is responsible for
+ * defining its own TAS_SPIN as well. (Note that we currently do not have any
+ * platforms that don't define TAS.)
+ */
#if !defined(TAS_SPIN)
+#if defined(TAS)
#define TAS_SPIN(lock) TAS(lock)
-#endif /* TAS_SPIN */
+#else
+#error Neither TAS nor TAS_SPIN defined on this platform. Please report this to [email protected].
+#endif /* TAS */
+#endif /* ! TAS_SPIN */
/*
--
2.50.1 (Apple Git-155)
--JmXY/Yn6swRVDyih--
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