linux-kernel - Re: [RFC][PATCH 1/3] locking: Replace smp_cond_acquire with smp_cond_load

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Date:	Mon, 4 Apr 2016 14:20:50 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>, <will.deacon@....com>,
	<mingo@...hat.com>, <paulmck@...ux.vnet.ibm.com>,
	<boqun.feng@...il.com>, <torvalds@...ux-foundation.org>,
	<dave@...olabs.net>
Subject: Re: [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@....com>
> Suggested-by: Linus Torvalds<torvalds@...ux-foundation.org>
> Signed-off-by: Peter Zijlstra (Intel)<peterz@...radead.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