Fix deadlock danger when atomic ops are done under spinlock.

This was a danger only for --disable-spinlocks in combination with
atomic operations unsupported by the current platform.

While atomics.c was careful to signal that a separate semaphore ought
to be used when spinlock emulation is active, spin.c didn't actually
implement that mechanism. That's my (Andres') fault, it seems to have
gotten lost during the development of the atomic operations support.

Fix that issue and add test for nesting atomic operations inside a
spinlock.

Author: Andres Freund
Discussion: https://postgr.es/m/20200605023302.g6v3ydozy5txifji@alap3.anarazel.de
Backpatch: 9.5-

1 files changed, 47 insertions, 0 deletions

diff --git a/src/test/regress/regress.c b/src/test/regress/regress.c
index 370d39e9fc2..ddfde1f2916 100644
--- a/src/test/regress/regress.c
+++ b/src/test/regress/regress.c
@@ -1159,6 +1159,51 @@ test_spinlock(void)
 #endif
 }
 
+/*
+ * Verify that performing atomic ops inside a spinlock isn't a
+ * problem. Realistically that's only going to be a problem when both
+ * --disable-spinlocks and --disable-atomics are used, but it's cheap enough
+ * to just always test.
+ *
+ * The test works by initializing enough atomics that we'd conflict if there
+ * were an overlap between a spinlock and an atomic by holding a spinlock
+ * while manipulating more than NUM_SPINLOCK_SEMAPHORES atomics.
+ *
+ * NUM_TEST_ATOMICS doesn't really need to be more than
+ * NUM_SPINLOCK_SEMAPHORES, but it seems better to test a bit more
+ * extensively.
+ */
+static void
+test_atomic_spin_nest(void)
+{
+	slock_t lock;
+#define NUM_TEST_ATOMICS (NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES + 27)
+	pg_atomic_uint32 atomics32[NUM_TEST_ATOMICS];
+
+	SpinLockInit(&lock);
+
+	for (int i = 0; i < NUM_TEST_ATOMICS; i++)
+	{
+		pg_atomic_init_u32(&atomics32[i], 0);
+	}
+
+	/* just so it's not all zeroes */
+	for (int i = 0; i < NUM_TEST_ATOMICS; i++)
+	{
+		EXPECT_EQ_U32(pg_atomic_fetch_add_u32(&atomics32[i], i), 0);
+	}
+
+	/* test whether we can do atomic op with lock held */
+	SpinLockAcquire(&lock);
+	for (int i = 0; i < NUM_TEST_ATOMICS; i++)
+	{
+		EXPECT_EQ_U32(pg_atomic_fetch_sub_u32(&atomics32[i], i), i);
+		EXPECT_EQ_U32(pg_atomic_read_u32(&atomics32[i]), 0);
+	}
+	SpinLockRelease(&lock);
+}
+#undef NUM_TEST_ATOMICS
+
 PG_FUNCTION_INFO_V1(test_atomic_ops);
 Datum
 test_atomic_ops(PG_FUNCTION_ARGS)
@@ -1177,5 +1222,7 @@ test_atomic_ops(PG_FUNCTION_ARGS)
 	 */
 	test_spinlock();
 
+	test_atomic_spin_nest();
+
 	PG_RETURN_BOOL(true);
 }