perf: Simplify the ring-buffer code

#include <linux/perf_event.h>

#include <linux/vmalloc.h>

#include <linux/slab.h>

#include <linux/circ_buf.h>

#include "internal.h"

static bool perf_output_space(struct ring_buffer *rb, unsigned long tail,

			      unsigned long offset, unsigned long head)

{

	unsigned long sz = perf_data_size(rb);

	unsigned long mask = sz - 1;

	/*

	 * check if user-writable

	 * overwrite : over-write its own tail

	 * !overwrite: buffer possibly drops events.

	 */

	if (rb->overwrite)

		return true;

	/*

	 * verify that payload is not bigger than buffer

	 * otherwise masking logic may fail to detect

	 * the "not enough space" condition

	 */

	if ((head - offset) > sz)

		return false;

	offset = (offset - tail) & mask;

	head   = (head   - tail) & mask;

	if ((int)(head - offset) < 0)

		return false;

	return true;

}

static void perf_output_wakeup(struct perf_output_handle *handle)

{

	atomic_set(&handle->rb->poll, POLL_IN);

		tail = ACCESS_ONCE(rb->user_page->data_tail);

		smp_mb();

		offset = head = local_read(&rb->head);

		head += size;

		if (unlikely(!perf_output_space(rb, tail, offset, head)))

		if (!rb->overwrite &&

		    unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))

			goto fail;

		head += size;

	} while (local_cmpxchg(&rb->head, offset, head) != offset);

	if (head - local_read(&rb->wakeup) > rb->watermark)