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

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] The use of `__sync_val_compare_and_swap` introduces a compiler dependency on BPF ISA v3, breaking the `perf` tool build on older toolchains.
--

commit 30820839a2cfcfa987fbc2e0f79fa0e2f2cf0dc3
Author: Suchit Karunakaran <suchitkarunakaran@gmail.com>

perf/lock: Fix non-atomic max/time and min_time updates in contention_data

This commit replaces bare conditional assignments with a bpf_loop()-based CAS
retry loop to fix non-atomic updates of max_time and min_time.

> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index 96e7d853b9edf..5c8431be674ac 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
[ ... ]
> @@ -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);

Does this cause build failures on older toolchains?

The built-in __sync_val_compare_and_swap lowers to the BPF_CMPXCHG
instruction, which requires BPF CPU v3 (-mcpu=v3).

The perf build system compiles BPF skeletons using the host compiler's default
target features, without explicitly passing an -mcpu flag. On toolchains
where the default BPF target is v1 or v2 (such as Clang < 18), the compiler
may fail to generate the instruction and throw a fatal error (e.g.,
'hardware compare and swap not supported' or an unresolved libcall to
__sync_val_compare_and_swap_8).

> +			if (r == old_max)
> +				ctx->max_done = 1;
> +		}
> +	}
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260504135138.17050-1-suchitkarunakaran@gmail.com?part=1