From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Borkmann Subject: [PATCH bpf-next 3/3] bpf, libbpf: use proper barriers in perf ring buffer walk Date: Wed, 17 Oct 2018 16:41:56 +0200 Message-ID: <20181017144156.16639-4-daniel@iogearbox.net> References: <20181017144156.16639-1-daniel@iogearbox.net> Cc: peterz@infradead.org, paulmck@linux.vnet.ibm.com, will.deacon@arm.com, acme@redhat.com, yhs@fb.com, john.fastabend@gmail.com, netdev@vger.kernel.org, Daniel Borkmann To: alexei.starovoitov@gmail.com Return-path: Received: from www62.your-server.de ([213.133.104.62]:36538 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727405AbeJQWjn (ORCPT ); Wed, 17 Oct 2018 18:39:43 -0400 In-Reply-To: <20181017144156.16639-1-daniel@iogearbox.net> Sender: netdev-owner@vger.kernel.org List-ID: Add bpf_perf_read_head() and bpf_perf_write_tail() helpers to make it more clear in what context barriers are used here, and use smp_rmb() as well as smp_mb() barriers. Given libbpf is not restricted to x86-64 only, the compiler barrier needs to be replaced with smp_rmb(). Also the __sync_synchronize() emits mfence whereas faster lock + add can be used on x86-64 via smp_mb(). 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 --- tools/lib/bpf/libbpf.c | 25 +++++++++++++++++++------ 1 file changed, 19 insertions(+), 6 deletions(-) diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index bd71efc..e8ae8db 100644 --- a/tools/lib/bpf/libbpf.c +++ b/tools/lib/bpf/libbpf.c @@ -20,6 +20,7 @@ #include #include #include +#include #include #include #include @@ -2413,18 +2414,32 @@ int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr, return 0; } +static __u64 bpf_perf_read_head(struct perf_event_mmap_page *header) +{ + __u64 data_head = READ_ONCE(header->data_head); + + smp_rmb(); + return data_head; +} + +static void bpf_perf_write_tail(struct perf_event_mmap_page *header, + __u64 data_tail) +{ + smp_mb(); + header->data_tail = data_tail; +} + enum bpf_perf_event_ret 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 = bpf_perf_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 +2482,6 @@ bpf_perf_event_read_simple(void *mem, unsigned long size, data_tail += ehdr->size; } - __sync_synchronize(); /* smp_mb() */ - header->data_tail = data_tail; - + bpf_perf_write_tail(header, data_tail); return ret; } -- 2.9.5