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-

2 files changed, 123 insertions, 23 deletions

diff --git a/src/backend/storage/lmgr/spin.c b/src/backend/storage/lmgr/spin.c
index dfa47a723f..7c381fe2ae 100644
--- a/src/backend/storage/lmgr/spin.c
+++ b/src/backend/storage/lmgr/spin.c
@@ -31,8 +31,24 @@
 
 
 #ifndef HAVE_SPINLOCKS
+
+/*
+ * No TAS, so spinlocks are implemented as PGSemaphores.
+ */
+
+#ifndef HAVE_ATOMICS
+#define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES)
+#else
+#define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES)
+#endif /* DISABLE_ATOMICS */
+
 PGSemaphore SpinlockSemaArray;
-#endif
+
+#else							/* !HAVE_SPINLOCKS */
+
+#define NUM_EMULATION_SEMAPHORES 0
+
+#endif							/* HAVE_SPINLOCKS */
 
 /*
  * Report the amount of shared memory needed to store semaphores for spinlock
@@ -41,34 +57,19 @@ PGSemaphore SpinlockSemaArray;
 Size
 SpinlockSemaSize(void)
 {
-	return SpinlockSemas() * sizeof(PGSemaphoreData);
+	return NUM_EMULATION_SEMAPHORES * sizeof(PGSemaphoreData);
 }
 
-#ifdef HAVE_SPINLOCKS
-
 /*
  * Report number of semaphores needed to support spinlocks.
  */
 int
 SpinlockSemas(void)
 {
-	return 0;
+	return NUM_EMULATION_SEMAPHORES;
 }
-#else							/* !HAVE_SPINLOCKS */
 
-/*
- * No TAS, so spinlocks are implemented as PGSemaphores.
- */
-
-
-/*
- * Report number of semaphores needed to support spinlocks.
- */
-int
-SpinlockSemas(void)
-{
-	return NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES;
-}
+#ifndef HAVE_SPINLOCKS
 
 /*
  * Initialize semaphores.
@@ -85,20 +86,68 @@ SpinlockSemaInit(PGSemaphore spinsemas)
 }
 
 /*
- * s_lock.h hardware-spinlock emulation
+ * s_lock.h hardware-spinlock emulation using semaphores
+ *
+ * We map all spinlocks onto NUM_EMULATION_SEMAPHORES semaphores.  It's okay to
+ * map multiple spinlocks onto one semaphore because no process should ever
+ * hold more than one at a time.  We just need enough semaphores so that we
+ * aren't adding too much extra contention from that.
+ *
+ * There is one exception to the restriction of only holding one spinlock at a
+ * time, which is that it's ok if emulated atomic operations are nested inside
+ * spinlocks. To avoid the danger of spinlocks and atomic using the same sema,
+ * we make sure "normal" spinlocks and atomics backed by spinlocks use
+ * distinct semaphores (see the nested argument to s_init_lock_sema).
+ *
+ * slock_t is just an int for this implementation; it holds the spinlock
+ * number from 0..(NUM_EMULATION_SEMAPHORES - 1).
  */
 
+static inline void
+s_check_valid(int lockndx)
+{
+	if (lockndx < 0 || lockndx >= NUM_EMULATION_SEMAPHORES)
+		elog(ERROR, "invalid spinlock number2: %d", lockndx);
+}
+
 void
 s_init_lock_sema(volatile slock_t *lock, bool nested)
 {
 	static uint32 counter = 0;
-
-	*lock = (++counter) % NUM_SPINLOCK_SEMAPHORES;
+	uint32		offset;
+	uint32		sema_total;
+	uint32		idx;
+
+	if (nested)
+	{
+		/*
+		 * To allow nesting atomics inside spinlocked sections, use a
+		 * different spinlock. See comment above.
+		 */
+		offset = NUM_SPINLOCK_SEMAPHORES;
+		sema_total = NUM_ATOMICS_SEMAPHORES;
+	}
+	else
+	{
+		offset = 0;
+		sema_total = NUM_SPINLOCK_SEMAPHORES;
+	}
+
+	idx = (counter++ % sema_total) + offset;
+
+	/* double check we did things correctly */
+	s_check_valid(idx);
+
+	*lock = idx;
 }
 
 void
 s_unlock_sema(volatile slock_t *lock)
 {
+	int			lockndx = *lock;
+
+	s_check_valid(lockndx);
+
 	PGSemaphoreUnlock(&SpinlockSemaArray[*lock]);
 }
 
@@ -113,8 +162,12 @@ s_lock_free_sema(volatile slock_t *lock)
 int
 tas_sema(volatile slock_t *lock)
 {
+	int			lockndx = *lock;
+
+	s_check_valid(lockndx);
+
 	/* Note that TAS macros return 0 if *success* */
-	return !PGSemaphoreTryLock(&SpinlockSemaArray[*lock]);
+	return !PGSemaphoreTryLock(&SpinlockSemaArray[lockndx]);
 }
 
 #endif   /* !HAVE_SPINLOCKS */
diff --git a/src/test/regress/regress.c b/src/test/regress/regress.c
index f6e337741b..36e8bc72a1 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);
 }