Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Apr 25, 2015 at 12:56:03PM -0700, Paul E. McKenney wrote:
>     smp: Make control dependencies work on Alpha, improve documentation
>     
>     The current formulation of control dependencies fails on DEC Alpha, which
>     does not respect dependencies of any kind unless an explicit memory is

 + barrier ?

>     provided.  This means that the current fomulation of control dependencies
>     fails on Alpha.  This commit therefore creates a READ_ONCE_CTRL() that
>     has the same overhead on non-Alpha systems, but causes Alpha to produce
>     the needed ordering.  This commit also applies READ_ONCE_CTRL() to the
>     one known use of control dependencies.
>     
>     Use of READ_ONCE_CTRL() also has the beneficial effect of adding a bit
>     of self-documentation to control dependencies.
>     
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 1b45e4a0519b..a57eacde2b84 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -264,6 +264,22 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>  #define WRITE_ONCE(x, val) \
>  	({ typeof(x) __val = (val); __write_once_size(&(x), &__val, sizeof(__val)); __val; })
>  
> +/**
> + * READ_ONCE_CTRL - Read a value heading a control dependency
> + * @x: The value to be read, heading the control dependency
> + *
> + * Control dependencies are tricky.  See Documentation/memory-barriers.txt
> + * for important information on how to use them.  Note that in many cases,
> + * use of smp_load_acquire() will be much simpler.  Control dependencies
> + * should be avoided except on the hottest of hotpaths.
> + */
> +#define READ_ONCE_CTRL(x) \
> +({ \
> +	typeof(x) __val = READ_ONCE(x); \
> +	smp_read_barrier_depends(); /* Enforce control dependency. */ \
> +	__val; \
> +})
> +
>  #endif /* __KERNEL__ */
>  
>  #endif /* __ASSEMBLY__ */

We mostly try and align the \ for multi-line macros.

> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index eadb95ce7aac..67548d5de4cb 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -141,7 +141,7 @@ int perf_output_begin(struct perf_output_handle *handle,
>  	perf_output_get_handle(handle);
>  
>  	do {
> -		tail = ACCESS_ONCE(rb->user_page->data_tail);
> +		tail = READ_ONCE_CTRL(rb->user_page->data_tail);
>  		offset = head = local_read(&rb->head);
>  		if (!rb->overwrite &&
>  		    unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))

Right. I could not remember any other current usage.