1#ifndef _TOOLS_LINUX_RING_BUFFER_H_
2#define _TOOLS_LINUX_RING_BUFFER_H_
3
4#include <asm/barrier.h>
5#include <linux/perf_event.h>
6
7/*
8 * Contract with kernel for walking the perf ring buffer from
9 * user space requires the following barrier pairing (quote
10 * from kernel/events/ring_buffer.c):
11 *
12 *   Since the mmap() consumer (userspace) can run on a
13 *   different CPU:
14 *
15 *   kernel                             user
16 *
17 *   if (LOAD ->data_tail) {            LOAD ->data_head
18 *                      (A)             smp_rmb()       (C)
19 *      STORE $data                     LOAD $data
20 *      smp_wmb()       (B)             smp_mb()        (D)
21 *      STORE ->data_head               STORE ->data_tail
22 *   }
23 *
24 *   Where A pairs with D, and B pairs with C.
25 *
26 *   In our case A is a control dependency that separates the
27 *   load of the ->data_tail and the stores of $data. In case
28 *   ->data_tail indicates there is no room in the buffer to
29 *   store $data we do not.
30 *
31 *   D needs to be a full barrier since it separates the data
32 *   READ from the tail WRITE.
33 *
34 *   For B a WMB is sufficient since it separates two WRITEs,
35 *   and for C an RMB is sufficient since it separates two READs.
36 *
37 * Note, instead of B, C, D we could also use smp_store_release()
38 * in B and D as well as smp_load_acquire() in C.
39 *
40 * However, this optimization does not make sense for all kernel
41 * supported architectures since for a fair number it would
42 * resolve into READ_ONCE() + smp_mb() pair for smp_load_acquire(),
43 * and smp_mb() + WRITE_ONCE() pair for smp_store_release().
44 *
45 * Thus for those smp_wmb() in B and smp_rmb() in C would still
46 * be less expensive. For the case of D this has either the same
47 * cost or is less expensive, for example, due to TSO x86 can
48 * avoid the CPU barrier entirely.
49 */
50
51static inline u64 ring_buffer_read_head(struct perf_event_mmap_page *base)
52{
53/*
54 * Architectures where smp_load_acquire() does not fallback to
55 * READ_ONCE() + smp_mb() pair.
56 */
57#if defined(__x86_64__) || defined(__aarch64__) || defined(__powerpc64__) || \
58    defined(__ia64__) || defined(__sparc__) && defined(__arch64__)
59	return smp_load_acquire(&base->data_head);
60#else
61	u64 head = READ_ONCE(base->data_head);
62
63	smp_rmb();
64	return head;
65#endif
66}
67
68static inline void ring_buffer_write_tail(struct perf_event_mmap_page *base,
69					  u64 tail)
70{
71	smp_store_release(&base->data_tail, tail);
72}
73
74#endif /* _TOOLS_LINUX_RING_BUFFER_H_ */
75