linux-kernel - Re: [PATCH v2 bpf-next 1/3] perf: enable branch record for software events

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Message-ID: <20210830102258.GI4353@worktop.programming.kicks-ass.net>
Date:   Mon, 30 Aug 2021 12:22:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        mingo@...hat.com, kjain@...ux.ibm.com, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 1/3] perf: enable branch record for software
 events

On Thu, Aug 26, 2021 at 03:13:04PM -0700, Song Liu wrote:
> +int dummy_perf_snapshot_branch_stack(struct perf_branch_snapshot *br_snapshot);
> +
> +DECLARE_STATIC_CALL(perf_snapshot_branch_stack, dummy_perf_snapshot_branch_stack);
> +
>  #endif /* _LINUX_PERF_EVENT_H */
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 011cc5069b7ba..c53fe90e630ac 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -13437,3 +13437,6 @@ struct cgroup_subsys perf_event_cgrp_subsys = {
>  	.threaded	= true,
>  };
>  #endif /* CONFIG_CGROUP_PERF */
> +
> +DEFINE_STATIC_CALL_NULL(perf_snapshot_branch_stack,
> +			dummy_perf_snapshot_branch_stack);

This isn't right...

The whole dummy_perf_snapshot_branch_stack() thing is a declaration only
and used as a typedef. Also, DEFINE_STATIC_CALL_NULL() and
static_call_cond() rely on a void return value, which it doesn't have.

Did you want:

  DECLARE_STATIC_CALL(perf_snapshot_branch_stack, void (*)(struct perf_branch_snapshot *));

  DEFINE_STATIC_CALL_NULL(perf_snapshot_branch_stack, void (*)(struct perf_branch_snapshot *));

  static_call_cond(perf_snapshot_branch_stack)(...);

*OR*, do you actually need that return value, in which case you're
probably looking for:

  DECLARE_STATIC_CALL(perf_snapshot_branch_stack, int (*)(struct perf_branch_snapshot *));

  DEFINE_STATIC_CALL_RET0(perf_snapshot_branch_stack, int (*)(struct perf_branch_snapshot *));

  ret = static_call(perf_snapshot_branch_stack)(...);

?