Fix generic read and write barriers for Clang.HEADmaster

generic-gcc.h maps our read and write barriers to C11 acquire and
release fences using compiler builtins, for platforms where we don't
have our own hand-rolled assembler.  This is apparently enough for GCC,
but the C11 memory model is only defined in terms of atomic accesses,
and our barriers for non-atomic, non-volatile accesses were not always
respected under Clang's stricter interpretation of the standard.

This explains the occasional breakage observed on new RISC-V + Clang
animal greenfly in lock-free PgAioHandle manipulation code containing a
repeating pattern of loads and read barriers.  The problem can also be
observed in code generated for MIPS and LoongAarch, though we aren't
currently testing those with Clang, and on x86, though we use our own
assembler there.  The scariest aspect is that we use the generic version
on very common ARM systems, but it doesn't seem to reorder the relevant
code there (or we'd have debugged this long ago).

Fix by inserting an explicit compiler barrier.  It expands to an empty
assembler block declared to have memory side-effects, so registers are
flushed and reordering is prevented.  In those respects this is like the
architecture-specific assembler versions, but the compiler is still in
charge of generating the appropriate fence instruction.  Done for write
barriers on principle, though concrete problems have only been observed
with read barriers.

Reported-by: Alexander Lakhin <[email protected]>
Tested-by: Alexander Lakhin <[email protected]>
Discussion: https://postgr.es/m/d79691be-22bd-457d-9d90-18033b78c40a%40gmail.com
Backpatch-through: 13

1 files changed, 10 insertions, 2 deletions

diff --git a/src/include/port/atomics/generic-gcc.h b/src/include/port/atomics/generic-gcc.h
index e7dfad4f0d5..a0751f2286a 100644
--- a/src/include/port/atomics/generic-gcc.h
+++ b/src/include/port/atomics/generic-gcc.h
@@ -44,12 +44,20 @@
 
 #if !defined(pg_read_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT32_CAS)
 /* acquire semantics include read barrier semantics */
-#		define pg_read_barrier_impl()		__atomic_thread_fence(__ATOMIC_ACQUIRE)
+#		define pg_read_barrier_impl() do \
+{ \
+	pg_compiler_barrier_impl(); \
+	__atomic_thread_fence(__ATOMIC_ACQUIRE); \
+} while (0)
 #endif
 
 #if !defined(pg_write_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT32_CAS)
 /* release semantics include write barrier semantics */
-#		define pg_write_barrier_impl()		__atomic_thread_fence(__ATOMIC_RELEASE)
+#		define pg_write_barrier_impl() do \
+{ \
+	pg_compiler_barrier_impl(); \
+	__atomic_thread_fence(__ATOMIC_RELEASE); \
+} while (0)
 #endif