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Sat, 30 May 2026 12:52:41 -0700 (PDT) Received: from archlinux ([205.254.163.119]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2bf23c4dd4csm58343345ad.78.2026.05.30.12.52.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 30 May 2026 12:52:40 -0700 (PDT) From: Suchit Karunakaran To: peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, james.clark@linaro.org Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Suchit Karunakaran Subject: [PATCH 1/2] perf/lock: Fix non-atomic max/time and min_time updates in contention_data Date: Sun, 31 May 2026 01:22:30 +0530 Message-ID: <20260530195230.69382-1-suchitkarunakaran@gmail.com> X-Mailer: git-send-email 2.54.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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. Acked-by: Namhyung Kim Signed-off-by: Suchit Karunakaran --- .../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