[PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types

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ACCESS_ONCE does not work reliably on non-scalar types. For
example gcc 4.6 and 4.7 might remove the volatile tag for such
accesses during the SRA (scalar replacement of aggregates) steps.
see  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

This patch is based on an initial proof-of-concept from Linus
Torvalds that causes compile errors for such accesses.

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
 include/linux/compiler.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index d5ad7b1..8a92c93 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -377,8 +377,18 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
  * merging, or refetching absolutely anything at any time.  Its main intended
  * use is to mediate communication between process-level code and irq/NMI
  * handlers, all running on the same CPU.
+ *
+ * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
+ * on a union member will work as long as the size of the member matches the
+ * size of the union and the size is smaller than word size. See the x86
+ * spinlock implementation as an example. For other cases like page table
+ * structures, a barrier might be a good alternative.
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define get_scalar_volatile_pointer(x) ({ \
+	typeof(x) *__p = &(x); \
+	volatile typeof(x) *__vp = __p; \
+	(void)(long)*__p; __vp; })
+#define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x))
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */

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