[PATCH 1/2] perf/lock: Fix non-atomic max/time and min_time updates in contention_data

[Suchit Karunakaran <suchitkarunakaran@gmail.com>]

The update_contention_data() had a FIXME noting that max_time and
min_time updates lacked atomicity. Two CPUs could simultaneously
read a stale value, pass the comparison check and race on the
write-back, with the smaller value potentially overwriting the
larger one and silently corrupting the statistics.

Fix this by replacing the bare conditional assignments with a
bpf_loop()-based CAS retry loop. Each field tracks its own
convergence independently via max_done/min_done flags in cas_ctx,
so a successful CAS on one field is never retried even if the
other field needs more attempts.

Signed-off-by: Suchit Karunakaran <suchitkarunakaran@gmail.com>
---
 .../perf/util/bpf_skel/lock_contention.bpf.c  | 50 +++++++++++++++++--
 1 file changed, 45 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 96e7d853b9ed..5c8431be674a 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -175,6 +175,13 @@ struct mm_struct___new {
 	struct rw_semaphore mmap_lock;
 } __attribute__((preserve_access_index));
 
+struct cas_ctx {
+	struct contention_data *data;
+	u64 duration;
+	int max_done;
+	int min_done;
+};
+
 extern struct kmem_cache *bpf_get_kmem_cache(u64 addr) __ksym __weak;
 
 /* control flags */
@@ -486,16 +493,49 @@ static inline s32 get_owner_stack_id(u64 *stacktrace)
 	return -1;
 }
 
+static long cas_min_max_cb(u64 idx, void *arg)
+{
+	struct cas_ctx *ctx = arg;
+
+	if (!ctx->max_done) {
+		u64 old_max = ctx->data->max_time;
+		if (old_max >= ctx->duration) {
+			ctx->max_done = 1;
+		} else {
+			u64 r = __sync_val_compare_and_swap(
+				&ctx->data->max_time, old_max, ctx->duration);
+			if (r == old_max)
+				ctx->max_done = 1;
+		}
+	}
+
+	if (!ctx->min_done) {
+		u64 old_min = ctx->data->min_time;
+		if (old_min <= ctx->duration) {
+			ctx->min_done = 1;
+		} else {
+			u64 r = __sync_val_compare_and_swap(
+				&ctx->data->min_time, old_min, ctx->duration);
+			if (r == old_min)
+				ctx->min_done = 1;
+		}
+	}
+
+	return (ctx->max_done && ctx->min_done) ? 1 : 0;
+}
+
 static inline void update_contention_data(struct contention_data *data, u64 duration, u32 count)
 {
 	__sync_fetch_and_add(&data->total_time, duration);
 	__sync_fetch_and_add(&data->count, count);
 
-	/* FIXME: need atomic operations */
-	if (data->max_time < duration)
-		data->max_time = duration;
-	if (data->min_time > duration)
-		data->min_time = duration;
+	struct cas_ctx ctx = {
+		.data     = data,
+		.duration = duration,
+		.max_done = 0,
+		.min_done = 0,
+	};
+	bpf_loop(64, cas_min_max_cb, &ctx, 0);
 }
 
 static inline void update_owner_stat(u32 id, u64 duration, u32 flags)
-- 
2.54.0