Re: spinlock support on loongarch64 - Mailing list pgsql-hackers

Andres Freund <andres@anarazel.de> writes:
> On 2022-11-02 11:37:35 -0400, Tom Lane wrote:
>> I wonder if we shouldn't just do that (ie, try to use
>> __sync_lock_test_and_set) as a generic fallback on any unsupported
>> architecture.  We could get rid of the separate stanza for RISC-V
>> that way.  The main thing that an arch-specific stanza could bring
>> is knowledge of the best data type width to use for a spinlock;
>> but I don't see a big problem with defaulting to "int".  We can
>> always add arch-specific stanzas for any machines where that's
>> shown to be a seriously poor choice.

> Yes, please. It might not be perfect for all architectures, and it might not
> be good for some very old architectures. But for anything new it'll be vastly
> better than not having spinlocks at all.

So about like this, then.

            regards, tom lane

diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index 4225d9b7fc..d3972f61fc 100644
--- a/src/include/storage/s_lock.h
+++ b/src/include/storage/s_lock.h
@@ -293,29 +293,6 @@ spin_delay(void)
 #endif     /* __arm__ || __arm || __aarch64__ */


-/*
- * RISC-V likewise uses __sync_lock_test_and_set(int *, int) if available.
- */
-#if defined(__riscv)
-#ifdef HAVE_GCC__SYNC_INT32_TAS
-#define HAS_TEST_AND_SET
-
-#define TAS(lock) tas(lock)
-
-typedef int slock_t;
-
-static __inline__ int
-tas(volatile slock_t *lock)
-{
-    return __sync_lock_test_and_set(lock, 1);
-}
-
-#define S_UNLOCK(lock) __sync_lock_release(lock)
-
-#endif     /* HAVE_GCC__SYNC_INT32_TAS */
-#endif     /* __riscv */
-
-
 /* S/390 and S/390x Linux (32- and 64-bit zSeries) */
 #if defined(__s390__) || defined(__s390x__)
 #define HAS_TEST_AND_SET
@@ -716,6 +693,52 @@ spin_delay(void)
 #endif    /* !defined(HAS_TEST_AND_SET) */


+/*
+ * Last-ditch try.
+ *
+ * If we have no platform-specific knowledge, but we found that the compiler
+ * provides __sync_lock_test_and_set(), use that.  Prefer the int-width
+ * version over the char-width version if we have both, on the rather dubious
+ * grounds that that's known to be more likely to work in the ARM ecosystem.
+ * (But we dealt with ARM above.)
+ */
+#if !defined(HAS_TEST_AND_SET)
+
+#if defined(HAVE_GCC__SYNC_INT32_TAS)
+#define HAS_TEST_AND_SET
+
+#define TAS(lock) tas(lock)
+
+typedef int slock_t;
+
+static __inline__ int
+tas(volatile slock_t *lock)
+{
+    return __sync_lock_test_and_set(lock, 1);
+}
+
+#define S_UNLOCK(lock) __sync_lock_release(lock)
+
+#elif defined(HAVE_GCC__SYNC_CHAR_TAS)
+#define HAS_TEST_AND_SET
+
+#define TAS(lock) tas(lock)
+
+typedef char slock_t;
+
+static __inline__ int
+tas(volatile slock_t *lock)
+{
+    return __sync_lock_test_and_set(lock, 1);
+}
+
+#define S_UNLOCK(lock) __sync_lock_release(lock)
+
+#endif     /* HAVE_GCC__SYNC_INT32_TAS */
+
+#endif    /* !defined(HAS_TEST_AND_SET) */
+
+
 /* 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
configureusing --disable-spinlocks.  However, performance will be poor.  Please report this to
pgsql-bugs@lists.postgresql.org.