----- Original Message -----
> From: "Hannes Frederic Sowa" <han...@stressinduktion.org>
> To: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>, "Eric Dumazet"
> <eric.duma...@gmail.com>, "Josh Triplett"
> <j...@joshtriplett.org>, linux-kernel@vger.kernel.org, mi...@kernel.org,
> la...@cn.fujitsu.com, dipan...@in.ibm.com,
> a...@linux-foundation.org, "mathieu desnoyers"
> <mathieu.desnoy...@efficios.com>, n...@us.ibm.com, t...@linutronix.de,
> pet...@infradead.org, rost...@goodmis.org, dhowe...@redhat.com,
> eduma...@google.com, dar...@dvhart.com,
> fweis...@gmail.com, s...@mit.edu, "David S. Miller" <da...@davemloft.net>,
> "Alexey Kuznetsov" <kuz...@ms2.inr.ac.ru>,
> "James Morris" <jmor...@namei.org>, "Hideaki YOSHIFUJI"
> <yoshf...@linux-ipv6.org>, "Patrick McHardy"
> <ka...@trash.net>, net...@vger.kernel.org
> Sent: Saturday, October 12, 2013 1:37:34 PM
> 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"); \
AFAIU, ACCESS_ONCE() is not meant to ensure atomicity of load/store, but rather
merely ensures that the compiler will not merge nor refetch accesses. I don't
think the assert check you propose is appropriate with respect to the
ACCESS_ONCE() semantic.
Thanks,
Mathieu
> + (volatile typeof(x) *)&(x); \
> +}))
>
> /* Ignore/forbid kprobes attach on very low level functions marked by this
> attribute: */
> #ifdef CONFIG_KPROBES
>
>
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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