[PATCH bpf-next v2 2/2] bpf, libbpf: use correct barriers in perf ring buffer walk

[Daniel Borkmann]

Given libbpf is a generic library and not restricted to x86-64 only,
the compiler barrier in bpf_perf_event_read_simple() after fetching
the head needs to be replaced with smp_rmb() at minimum. Also, writing
out the tail we should use WRITE_ONCE() to avoid store tearing.

Now that we have the logic in place in ring_buffer_read_head() and
ring_buffer_write_tail() helper also used by perf tool which would
select the correct and best variant for a given architecture (e.g.
x86-64 can avoid CPU barriers entirely), make use of these in order
to fix bpf_perf_event_read_simple().

Fixes: d0cabbb021be ("tools: bpf: move the event reading loop to libbpf")
Fixes: 39111695b1b8 ("samples: bpf: add bpf_perf_event_output example")
Signed-off-by: Daniel Borkmann <dan...@iogearbox.net>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
Cc: Will Deacon <will.dea...@arm.com>
Cc: Arnaldo Carvalho de Melo <a...@redhat.com>
---
 tools/lib/bpf/libbpf.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index bd71efc..0c21355 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -27,6 +27,7 @@
 #include <linux/list.h>
 #include <linux/limits.h>
 #include <linux/perf_event.h>
+#include <linux/ring_buffer.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/vfs.h>
@@ -2418,13 +2419,12 @@ bpf_perf_event_read_simple(void *mem, unsigned long 
size,
                           unsigned long page_size, void **buf, size_t *buf_len,
                           bpf_perf_event_print_t fn, void *priv)
 {
-       volatile struct perf_event_mmap_page *header = mem;
+       struct perf_event_mmap_page *header = mem;
+       __u64 data_head = ring_buffer_read_head(header);
        __u64 data_tail = header->data_tail;
-       __u64 data_head = header->data_head;
        int ret = LIBBPF_PERF_EVENT_ERROR;
        void *base, *begin, *end;
 
-       asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
        if (data_head == data_tail)
                return LIBBPF_PERF_EVENT_CONT;
 
@@ -2467,8 +2467,6 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
                data_tail += ehdr->size;
        }
 
-       __sync_synchronize(); /* smp_mb() */
-       header->data_tail = data_tail;
-
+       ring_buffer_write_tail(header, data_tail);
        return ret;
 }
-- 
2.9.5