Commit a64af0ef authored Oct 19, 2018 by Daniel Borkmann Committed by Alexei Starovoitov Oct 19, 2018

bpf, libbpf: use correct barriers in perf ring buffer walk



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: d0cabbb0 ("tools: bpf: move the event reading loop to libbpf")
Fixes: 39111695 ("samples: bpf: add bpf_perf_event_output example")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent 09d62154

tools/lib/bpf/libbpf.c

+4 −6

Original line number	Diff line number	Diff line
		@@ -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;
		}

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