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 19:42:18 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	"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, 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

----- Original Message -----
> 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" <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
> 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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