Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()

[Will Deacon <will.deacon@arm.com>]

Hi Peter,

On Thu, Dec 03, 2015 at 01:40:13PM +0100, Peter Zijlstra wrote:
> Introduce smp_cond_acquire() which combines a control dependency and a
> read barrier to form acquire semantics.
> 
> This primitive has two benefits:
>  - it documents control dependencies,
>  - its typically cheaper than using smp_load_acquire() in a loop.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/compiler.h   |   17 +++++++++++++++++
>  kernel/locking/qspinlock.c |    3 +--
>  kernel/sched/core.c        |    8 +-------
>  kernel/sched/sched.h       |    2 +-
>  4 files changed, 20 insertions(+), 10 deletions(-)
> 
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -299,6 +299,23 @@ static __always_inline void __write_once
>  	__u.__val;					\
>  })
>  
> +/**
> + * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
> + * @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.
> + *
> + * 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)
> +
>  #endif /* __KERNEL__ */
>  
>  #endif /* __ASSEMBLY__ */
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -446,8 +446,7 @@ void queued_spin_lock_slowpath(struct qs
>  	if ((val = pv_wait_head_or_lock(lock, node)))
>  		goto locked;
>  
> -	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
> -		cpu_relax();
> +	smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));

I think we spoke about this before, but what would work really well for
arm64 here is if we could override smp_cond_acquire in such a way that
the atomic_read could be performed explicitly in the macro. That would
allow us to use an LDXR to set the exclusive monitor, which in turn
means we can issue a WFE and get a cheap wakeup when lock->val is
actually modified.

With the current scheme, there's not enough information expressed in the
"cond" parameter to perform this optimisation.

Cheers,

Will