linux-kernel - Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access

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Date:	Sat, 12 Oct 2013 19:37:34 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive

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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