Re: [PATCH bpf-next 2/3] tools, perf: use smp_{rmb,mb} barriers instead of {rmb,mb}

diff --git a/tools/include/linux/ring_buffer.h b/tools/include/linux/ring_buffer.h
new file mode 100644
index 0000000..48200e0
--- /dev/null
+++ b/tools/include/linux/ring_buffer.h

@@ -0,0 +1,69 @@
+#ifndef _TOOLS_LINUX_RING_BUFFER_H_
+#define _TOOLS_LINUX_RING_BUFFER_H_
+
+#include <linux/compiler.h>
+#include <asm/barrier.h>
+
+/*
+ * Below barriers pair as follows (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 makes sense not for all architectures since 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 (total store order),
+ * 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_ */