Commit 6a070573 authored Oct 20, 2023 by Namhyung Kim

perf lock contention: Check race in tstamp elem creation



When pelem is NULL, it'd create a new entry with zero data.  But it
might be preempted by IRQ/NMI just before calling bpf_map_update_elem()
then there's a chance to call it twice for the same pid.  So it'd be
better to use BPF_NOEXIST flag and check the return value to prevent
the race.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Song Liu <song@kernel.org>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20231020204741.1869520-2-namhyung@kernel.org

parent d99317f2

tools/perf/util/bpf_skel/lock_contention.bpf.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -328,7 +328,11 @@ int contention_begin(u64 *ctx)
		if (pelem == NULL) {
		struct tstamp_data zero = {};

		bpf_map_update_elem(&tstamp, &pid, &zero, BPF_ANY);
		if (bpf_map_update_elem(&tstamp, &pid, &zero, BPF_NOEXIST) < 0) {
		__sync_fetch_and_add(&task_fail, 1);
		return 0;
		}

		pelem = bpf_map_lookup_elem(&tstamp, &pid);
		if (pelem == NULL) {
		__sync_fetch_and_add(&task_fail, 1);