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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#include <linux/perf_event.h>6 7/*8 * Contract with kernel for walking the perf ring buffer from9 * user space requires the following barrier pairing (quote10 * from kernel/events/ring_buffer.c):11 *12 * Since the mmap() consumer (userspace) can run on a13 * different CPU:14 *15 * kernel user16 *17 * if (LOAD ->data_tail) { LOAD ->data_head18 * (A) smp_rmb() (C)19 * STORE $data LOAD $data20 * smp_wmb() (B) smp_mb() (D)21 * STORE ->data_head STORE ->data_tail22 * }23 *24 * Where A pairs with D, and B pairs with C.25 *26 * In our case A is a control dependency that separates the27 * load of the ->data_tail and the stores of $data. In case28 * ->data_tail indicates there is no room in the buffer to29 * store $data we do not.30 *31 * D needs to be a full barrier since it separates the data32 * 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 kernel41 * supported architectures since for a fair number it would42 * 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 still46 * be less expensive. For the case of D this has either the same47 * cost or is less expensive, for example, due to TSO x86 can48 * 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 to55 * READ_ONCE() + smp_mb() pair.56 */57#if defined(__x86_64__) || defined(__aarch64__) || defined(__powerpc64__) || \58 defined(__ia64__) || defined(__sparc__) && defined(__arch64__) || defined(__riscv)59 return smp_load_acquire(&base->data_head);60#else61 u64 head = READ_ONCE(base->data_head);62 63 smp_rmb();64 return head;65#endif66}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