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

Hello,

On Mon, May 04, 2026 at 05:23:37PM +0000, [email protected] wrote:
> 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 <[email protected]>
> 
> 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).

Hmm.. this looks like a real concern.  We could add -mcpu=v3 to the
compiler option, but then there would be compatibility issues with old
kernels.  It seems it's added in v5.1 kernel and the oldest longterm
support kernel version is 5.10.  So I think it's fine to add it.

Could you please update the build flag as well?  It can be a separate
commit.

Thanks,
Namhyung

> 
> 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/[email protected]?part=1