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Date: Sun, 13 Oct 2013 04:14:39 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>, 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 On Sat, Oct 12, 2013 at 07:42:18PM +0000, Mathieu Desnoyers wrote: > ----- Original Message ----- > > 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. I am with Mathieu on this one, at least unless there is some set of actual bugs already in the kernel that these length checks would find. /me wonders about structs of size 3, 5, 6, and 7... Thanx, Paul > 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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