#ifndef _TOOLS_LINUX_RING_BUFFER_H_#define _TOOLS_LINUX_RING_BUFFER_H_ #include <asm/barrier.h>#include <linux/perf_event.h> /* * Contract with kernel for walking the perf ring buffer from * user space requires the following barrier pairing (quote * from kernel/events/ring_buffer.c): * *   Since the mmap() consumer (userspace) can run on a *   different CPU: * *   kernel                             user * *   if (LOAD ->data_tail) {            LOAD ->data_head *                      (A)             smp_rmb()       (C) *      STORE $data                     LOAD $data *      smp_wmb()       (B)             smp_mb()        (D) *      STORE ->data_head               STORE ->data_tail *   } * *   Where A pairs with D, and B pairs with C. * *   In our case A is a control dependency that separates the *   load of the ->data_tail and the stores of $data. In case *   ->data_tail indicates there is no room in the buffer to *   store $data we do not. * *   D needs to be a full barrier since it separates the data *   READ from the tail WRITE. * *   For B a WMB is sufficient since it separates two WRITEs, *   and for C an RMB is sufficient since it separates two READs. * * Note, instead of B, C, D we could also use smp_store_release() * in B and D as well as smp_load_acquire() in C. * * However, this optimization does not make sense for all kernel * supported architectures since for a fair number it would * resolve into READ_ONCE() + smp_mb() pair for smp_load_acquire(), * and smp_mb() + WRITE_ONCE() pair for smp_store_release(). * * Thus for those smp_wmb() in B and smp_rmb() in C would still * be less expensive. For the case of D this has either the same * cost or is less expensive, for example, due to TSO x86 can * avoid the CPU barrier entirely. */ static inline u64 ring_buffer_read_head(struct perf_event_mmap_page *base){/* * Architectures where smp_load_acquire() does not fallback to * READ_ONCE() + smp_mb() pair. */#if defined(__x86_64__) || defined(__aarch64__) || defined(__powerpc64__) || \    defined(__ia64__) || defined(__sparc__) && defined(__arch64__)	return smp_load_acquire(&base->data_head);#else	u64 head = READ_ONCE(base->data_head); 	smp_rmb();	return head;#endif} static inline void ring_buffer_write_tail(struct perf_event_mmap_page *base,					  u64 tail){	smp_store_release(&base->data_tail, tail);} #endif /* _TOOLS_LINUX_RING_BUFFER_H_ */