linux-kernel - Re: [RFC] tracing: Add support for critical section event tracing

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Message-ID: <CAJWu+ooLtWNwhicoXjt-=-CuW72gD+-FhVYUM9tBf3JTkPJwKQ@mail.gmail.com>
Date:   Fri, 30 Jun 2017 14:03:14 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] tracing: Add support for critical section event tracing

Hi Steven,

Thanks a lot for the comments, I agree with all of them and had a
comment about one of them:

On Fri, Jun 30, 2017 at 5:51 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
[..]
> Are you not worried about recursion here? There's no protection.
> Wouldn't it be better to have:
>
>         if (!this_cpu_read(tracing_events_cpu))
>                 return;
>
>         trace_critical_end(ip, parent_ip);
>
>         this_cpu_write(tracing_events_cpu, 0);
>
> ?
>

I tried to go over some scenarios and I think it shouldn't be a
problem because we start the critical event only when either
interrupts are turned off while preemption is turned on, or preempt is
turned off while interrupts are turned on, and the fact that we call
the tracer while still in the critical section. Let me know if you had
a scenario in mind that can cause problems with this.

Anyway, I will rearrange the code like you suggested just to be extra safe,

Thanks,
Joel