Re: [lttng-dev] [PATCH] Add ACCESS_ONCE() to avoid compiler splitting assignments

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> As noted by Konstantin Khlebnikov, gcc can split assignment of
> constants to long variables (https://lkml.org/lkml/2013/1/15/141),
> though assignment of NULL (0) is OK.  Assuming that a gcc bug is
> fixed (http://gcc.gnu.org/bugzilla/attachment.cgi?id=29169&action=diff
> has a patch), making the store be volatile keeps gcc from splitting.
> 
> This commit therefore applies ACCESS_ONCE() to CMM_STORE_SHARED(),
> which is the underlying primitive used by rcu_assign_pointer().

Hi Paul,

I recognise that this is an issue in the Linux kernel, since a simple
store is used and expected to be performed atomically when aligned.
However, I think this does not affect liburcu, see below:

> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/urcu/system.h b/urcu/system.h
> index 2a45f22..7a1887e 100644
> --- a/urcu/system.h
> +++ b/urcu/system.h
> @@ -49,7 +49,7 @@
>   */
>  #define CMM_STORE_SHARED(x, v)		\
>  	({				\
> -		__typeof__(x) _v = _CMM_STORE_SHARED(x, v);	\
> +		__typeof__(x) CMM_ACCESS_ONCE(_v) = _CMM_STORE_SHARED(x, v);	\

Here, the macro "_CMM_STORE_SHARED(x, v)" is doing the actual store.
It stores v into "x". So adding a CMM_ACCESS_ONCE(_v), as you propose
here, is really only making sure the return value (usually unused),
located on the stack, is accessed with a volatile access, which does not
make much sense.

What really matters is the _CMM_STORE_SHARED() macro:

#define _CMM_STORE_SHARED(x, v) ({ CMM_ACCESS_ONCE(x) = (v); })

which already uses a volatile access for the store. So this seems to be
a case where our preemptive use of volatile for stores in addition to
loads made us bug-free for a gcc behavior unexpected at the time we
implemented this macro. Just a touch of paranoia seems to be a good
thing sometimes. ;-)

Thoughts ?

Thanks,

Mathieu

>  		cmm_smp_wmc();		\
>  		_v;			\
>  	})
> 
> 
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com