2023-10-20 20:47:57

by Namhyung Kim

Subject: [PATCH v3 2/3] 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 <[email protected]>
---
tools/perf/util/bpf_skel/lock_contention.bpf.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index b11179452e19..69d31fd77cd0 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -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);
--
2.42.0.655.g421f12c284-goog

2023-10-25 04:14:13

by Ian Rogers

Subject: Re: [PATCH v3 2/3] perf lock contention: Check race in tstamp elem creation

On Fri, Oct 20, 2023 at 1:47 PM Namhyung Kim <[email protected]> wrote:
>
> 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 <[email protected]>

Acked-by: Ian Rogers <[email protected]>

Thanks,
Ian
> ---
> tools/perf/util/bpf_skel/lock_contention.bpf.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index b11179452e19..69d31fd77cd0 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -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);
> --
> 2.42.0.655.g421f12c284-goog
>