[lttng-dev] liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ?

[Paul E. McKenney]

On Fri, Apr 16, 2021 at 03:30:53PM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 16, 2021, at 3:02 PM, paulmck paulmck at kernel.org wrote:
> [...]
> > 
> > If it can be done reasonably, I suggest also having some way for the
> > person building userspace RCU to say "I know what I am doing, so do
> > it with volatile rather than memory_order_consume."
> 
> Like so ?
> 
> #define CMM_ACCESS_ONCE(x) (*(__volatile__  __typeof__(x) *)&(x))
> #define CMM_LOAD_SHARED(p) CMM_ACCESS_ONCE(p)
> 
> /*
>  * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
>  * volatile access to implement rcu_dereference rather than
>  * memory_order_consume load from the C11/C++11 standards.
>  *
>  * This may improve performance on weakly-ordered architectures where
>  * the compiler implements memory_order_consume as a
>  * memory_order_acquire, which is stricter than required by the
>  * standard.
>  *
>  * Note that using volatile accesses for rcu_dereference may cause
>  * LTO to generate incorrectly ordered code starting from C11/C++11.
>  */
> 
> #ifdef URCU_DEREFERENCE_USE_VOLATILE
> # define rcu_dereference(x)     CMM_LOAD_SHARED(x)
> #else
> # if defined (__cplusplus)
> #  if __cplusplus >= 201103L
> #   include <atomic>
> #   define rcu_dereference(x)   ((std::atomic<__typeof__(x)>)(x)).load(std::memory_order_consume)
> #  else
> #   define rcu_dereference(x)   CMM_LOAD_SHARED(x)
> #  endif
> # else
> #  if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L)
> #   include <stdatomic.h>
> #   define rcu_dereference(x)   atomic_load_explicit(&(x), memory_order_consume)
> #  else
> #   define rcu_dereference(x)   CMM_LOAD_SHARED(x)
> #  endif
> # endif
> #endif

Looks good to me!

							Thanx, Paul