linux-kernel - Re: [PATCH 1/4] tracing: Use the perf recursion protection from trace event

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Date:	Sun, 22 Nov 2009 12:00:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH 1/4] tracing: Use the perf recursion protection from
 trace event

On Sun, 2009-11-22 at 05:21 +0100, Frederic Weisbecker wrote:
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index 718fa93..aba8227 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -3880,34 +3880,42 @@ static void perf_swevent_ctx_event(struct perf_event_context *ctx,
>         }
>  }
>  
> -static int *perf_swevent_recursion_context(struct perf_cpu_context *cpuctx)
> +/*
> + * Must be called with preemption disabled
> + */
> +int perf_swevent_get_recursion_context(int **recursion)
>  {
> +       struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context);
> +
>         if (in_nmi())
> -               return &cpuctx->recursion[3];
> +               *recursion = &cpuctx->recursion[3];
> +       else if (in_irq())
> +               *recursion = &cpuctx->recursion[2];
> +       else if (in_softirq())
> +               *recursion = &cpuctx->recursion[1];
> +       else
> +               *recursion = &cpuctx->recursion[0];
>  
> -       if (in_irq())
> -               return &cpuctx->recursion[2];
> +       if (**recursion)
> +               return -1;
>  
> -       if (in_softirq())
> -               return &cpuctx->recursion[1];
> +       (**recursion)++;
>  
> -       return &cpuctx->recursion[0];
> +       return 0;
>  }

You lost the barrier();

> -static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
> -                                   u64 nr, int nmi,
> -                                   struct perf_sample_data *data,
> -                                   struct pt_regs *regs)
> +void perf_swevent_put_recursion_context(int *recursion)
>  {
> -       struct perf_cpu_context *cpuctx = &get_cpu_var(perf_cpu_context);
> -       int *recursion = perf_swevent_recursion_context(cpuctx);
> -       struct perf_event_context *ctx;
> -
> -       if (*recursion)
> -               goto out;
> +       (*recursion)--;
> +}

And here as well.

Furthermore, its much cleaner if you simply use
get_cpu_var(perf_cpu_context) in get_recursion_context, and
put_cpu_var() in put_recursion_context.

That way you put the preempt_disable where it belongs, instead of:

> +static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
> +                                   u64 nr, int nmi,
> +                                   struct perf_sample_data *data,
> +                                   struct pt_regs *regs)
> +{
> +       int *recursion;
> +
> +       preempt_disable();
> +
> +       if (perf_swevent_get_recursion_context(&recursion))
> +               goto out;
> +
> +       __do_perf_sw_event(type, event_id, nr, nmi, data, regs);
>  
> +       perf_swevent_put_recursion_context(recursion);
>  out:
> -       put_cpu_var(perf_cpu_context);
> +       preempt_enable();
>  } 

mucking about like here, also it cleans up the code in that you don't
have to drag that recursion variable around like so.

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