Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive

[Hannes Frederic Sowa]

On Sat, Oct 12, 2013 at 06:43:45PM +0200, Hannes Frederic Sowa wrote:
> Regarding the volatile access, I hope that the C11 memory model
> and enhancements to the compiler will some day provide a better
> way to express the semantics of what is tried to express here
> (__atomic_store_n/__atomic_load_n with the accompanied memory model,
> which could be even weaker to what a volatile access would enfore
> now and could guarantee atomic stores/loads).

I just played around a bit more. Perhaps we could try to warn of silly
usages of ACCESS_ONCE():

--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -349,7 +349,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
  * use is to mediate communication between process-level code and irq/NMI
  * handlers, all running on the same CPU.
  */
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define ACCESS_ONCE(x) (*({						\
+		compiletime_assert(sizeof(typeof(x)) <= sizeof(typeof(&x)), \
+				   "ACCESS_ONCE likely not atomic");	\
+		(volatile typeof(x) *)&(x);				\
+}))
 
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
 #ifdef CONFIG_KPROBES

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