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 <paul...@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra (Intel) <pet...@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.
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