Messages in this thread | | | Date | Sun, 13 Oct 2013 22:11:02 +0200 | From | Hannes Frederic Sowa <> | Subject | Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive |
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On Sun, Oct 13, 2013 at 04:14:39AM -0700, Paul E. McKenney wrote: > 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.
I guess my wording of "ACCESS_ONCE likely not atomic" was misplaced. Something like volatile access to memory larger than the processor register size is probably not what you intended. Use atomics or proper locking. ;) And maybe that is not even correct.
> /me wonders about structs of size 3, 5, 6, and 7...
Checked a x86_64 allyesconfig build with sizes above pointer size and odd parity and nothing broke.
Greetings,
Hannes
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