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SubjectRe: [RFC][PATCH 1/3] locking: Replace smp_cond_acquire with smp_cond_load_acquire
On 04/04/2016 08:22 AM, Peter Zijlstra wrote:
> This new form allows using hardware assisted waiting.
>
> Requested-by: Will Deacon<will.deacon@arm.com>
> Suggested-by: Linus Torvalds<torvalds@linux-foundation.org>
> Signed-off-by: Peter Zijlstra (Intel)<peterz@infradead.org>
> ---
> include/linux/compiler.h | 25 +++++++++++++++++++------
> kernel/locking/qspinlock.c | 12 ++++++------
> kernel/sched/core.c | 8 ++++----
> kernel/sched/sched.h | 2 +-
> kernel/smp.c | 2 +-
> 5 files changed, 31 insertions(+), 18 deletions(-)
>
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -305,21 +305,34 @@ static __always_inline void __write_once
> })
>
> /**
> - * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
> + * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
> + * @ptr: pointer to the variable to wait on
> * @cond: boolean expression to wait for
> *
> * Equivalent to using smp_load_acquire() on the condition variable but employs
> * the control dependency of the wait to reduce the barrier on many platforms.
> *
> + * Due to C lacking lambda expressions we load the value of *ptr into a
> + * pre-named variable @VAL to be used in @cond.
> + *
> * The control dependency provides a LOAD->STORE order, the additional RMB
> * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
> * aka. ACQUIRE.
> */
> -#define smp_cond_acquire(cond) do { \
> - while (!(cond)) \
> - cpu_relax(); \
> - smp_rmb(); /* ctrl + rmb := acquire */ \
> -} while (0)
> +#ifndef smp_cond_load_acquire
> +#define smp_cond_load_acquire(ptr, cond_expr) ({ \
> + typeof(ptr) __PTR = (ptr); \
> + typeof(*ptr) VAL; \
> + for (;;) { \
> + VAL = READ_ONCE(*__PTR); \
> + if (cond_expr) \
> + break; \
> + cpu_relax(); \
> + } \
> + smp_rmb(); /* ctrl + rmb := acquire */ \
> + VAL; \
> +})
> +#endif

Using a predefined VAR seems a bit awkward as a reader of the code may
not know where VAR comes from. How about passing in a variable to hold
the latest value of (*ptr), e.g.

#ifndef smp_cond_load_acquire
#define smp_cond_load_acquire(ptr, var, cond_expr) do { \
typeof(ptr) __PTR = (ptr); \
for (;;) { \
var = READ_ONCE(*__PTR); \
if (cond_expr) \
break; \
cpu_relax(); \
} \
smp_rmb(); /* ctrl + rmb := acquire */ \
} while (0)
#endif

Cheers,
Longman

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