linux-kernel - Re: [PATCH] tracing: Make tracepoint lockdep check actually test something

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Message-ID: <20230310104458.4f3b1343@gandalf.local.home>
Date:   Fri, 10 Mar 2023 10:44:58 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH] tracing: Make tracepoint lockdep check actually test
 something

On Fri, 10 Mar 2023 17:27:58 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> This check has been introduced by commit 3a630178fd5f ("tracing: generate RCU
> warnings even when tracepoints are disabled"), and it also added a comment
> above this macro.
> 
>  * When lockdep is enabled, we make sure to always do the RCU portions of
>  * the tracepoint code, regardless of whether tracing is on. However,
>  * don't check if the condition is false, due to interaction with idle
>  * instrumentation. This lets us find RCU issues triggered with tracepoints
>  * even when this tracepoint is off. This code has no purpose other than
>  * poking RCU a bit.
> 
> I think at least the last sentence will be outdated by this fix.

Ah thanks, I forgot to update that part. What about:

  * When lockdep is enabled, we make sure to always test if RCU is
  * "watching" regardless if the tracepoint is enabled or not. Tracepoints
  * require RCU to be active, and it should always warn at the tracepoint
  * site if it is not watching, as it will need to be active when the
  * tracepoint is enabled.

?

-- Steve