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

Peter Zijlstra
2009-11-22T11:00:33Z

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.