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