linux-kernel - Re: [PATCH] trace/osnoise: fix event unhooking

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Date:   Mon, 3 Jan 2022 10:06:50 +0100
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Nikita Yushchenko <nikita.yushchenko@...tuozzo.com>
Cc:     linux-kernel@...r.kernel.org, kernel@...nvz.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] trace/osnoise: fix event unhooking

Hi Nikita

On 12/28/21 15:07, Nikita Yushchenko wrote:
> If start_per_cpu_kthreads() called from osnoise_workload_start() returns
> error, event hooks are left in broken state: unhook_irq_events() called
> but unhook_thread_events() and unhook_softirq_events() not called, and
> trace_osnoise_callback_enabled flag not cleared.
> 
> On the next tracer enable, hooks get not installed due to
> trace_osnoise_callback_enabled flag.
> 
> And on the further tracer disable an attempt to remove non-installed
> hooks happened, hitting a WARN_ON_ONCE() in tracepoint_remove_func().
> 
> Fix the error path by adding the missing part of cleanup.

Regarding the subject:

 - use tracing/ as subsystem (yeah, I also made this mistake in the original
   osnoise series).
 - use a capital after the "tracing/osnoise:"

Using your subject as example, it should be:
    tracing/osnoise: Fix event unhooking

Anyway, I suggest using something more precise, like:

  tracing/osnoise: Properly unhook events if start_per_cpu_kthreads() fails

or something like that.

> While at this, introduce osnoise_unhook_events() to avoid code
> duplication between this error path and notmal tracer disable.

s/notmal/normal/

> 
> Fixes: bce29ac9ce0b ("trace: Add osnoise tracer")
> Signed-off-by: Nikita Yushchenko <nikita.yushchenko@...tuozzo.com>
> ---
>  kernel/trace/trace_osnoise.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
> index 7520d43aed55..aa6f26612ccc 100644
> --- a/kernel/trace/trace_osnoise.c
> +++ b/kernel/trace/trace_osnoise.c
> @@ -2123,6 +2123,13 @@ static int osnoise_hook_events(void)
>  	return -EINVAL;
>  }
>  
> +static void osnoise_unhook_events(void)
> +{
> +	unhook_thread_events();
> +	unhook_softirq_events();
> +	unhook_irq_events();
> +}
> +
>  /*
>   * osnoise_workload_start - start the workload and hook to events
>   */
> @@ -2155,7 +2162,8 @@ static int osnoise_workload_start(void)
>  
>  	retval = start_per_cpu_kthreads();
>  	if (retval) {
> -		unhook_irq_events();
> +		trace_osnoise_callback_enabled = false;

we need a barrier here, like:

		/*
		 * Make sure that ftrace_nmi_enter/exit() see
		 * trace_osnoise_callback_enabled as false before continuing.
		 */
		barrier();

> +		osnoise_unhook_events();
>  		return retval;
>  	}
>  

Thanks!

-- Daniel