Hi!

On Fri, Dec 13, 2019 at 11:07:55PM +1100, Michael Ellerman wrote:

I tried this:

quoted

@@ -295,6 +296,23 @@ void __write_once_size(volatile void *p, void *res, int size)
  */
 #define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
 
+#else /* GCC_VERSION < 40800 */
+
+#define READ_ONCE_NOCHECK(x)						\
+({									\
+	typeof(x) __x = *(volatile typeof(x))&(x);			\

Didn't compile, needed:

	typeof(x) __x = *(volatile typeof(&x))&(x);			\

quoted

+	smp_read_barrier_depends();					\
+	__x;
+})

And that works for me. No extra stack check stuff.

I guess the question is does that version of READ_ONCE() implement the
read once semantics. Do we have a good way to test that?

The only differences are because of the early return in the generic
test_and_set_bit_lock():

No, there is another difference:

  30         ld      r10,560(r9)
  31         std     r10,104(r1)
  32         ld      r10,104(r1)
  33         andi.   r10,r10,1
  34         bne     <ext4_resize_begin_generic+0xd0>       29         bne     <ext4_resize_begin_ppc+0xd0>

The stack var is volatile, so it is read back immediately after writing
it, here.  This is a bad idea for performance, in general.


Segher