linux-kernel - Re: [PATCH RFC v5 3/6] srcu: Add notrace variant of srcu

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Message-Id: <20180501141817.GE26088@linux.vnet.ibm.com>
Date:   Tue, 1 May 2018 07:18:17 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Fenguang Wu <fengguang.wu@...el.com>,
        Baohong Liu <baohong.liu@...el.com>,
        Vedang Patel <vedang.patel@...el.com>, kernel-team@...roid.com
Subject: Re: [PATCH RFC v5 3/6] srcu: Add notrace variant of srcu_dereference

On Mon, Apr 30, 2018 at 06:42:01PM -0700, Joel Fernandes wrote:
> In this series, we are making lockdep use an rcuidle tracepoint. For
> this reason we need a notrace variant of srcu_dereference since
> otherwise we get lockdep splats since lockdep hooks may not have run
> yet. This patch adds the needed variant.
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Peter Zilstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Tom Zanussi <tom.zanussi@...ux.intel.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Thomas Glexiner <tglx@...utronix.de>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Fenguang Wu <fengguang.wu@...el.com>
> Cc: Baohong Liu <baohong.liu@...el.com>
> Cc: Vedang Patel <vedang.patel@...el.com>
> Cc: kernel-team@...roid.com
> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
> ---
>  include/linux/srcu.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index 2ec618979b20..a1c4947be877 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -135,6 +135,11 @@ static inline int srcu_read_lock_held(const struct srcu_struct *sp)
>   */
>  #define srcu_dereference(p, sp) srcu_dereference_check((p), (sp), 0)
> 
> +/**
> + * srcu_dereference_notrace - no tracing and no lockdep calls from here
> + */
> +#define srcu_dereference_notrace(p, sp) srcu_dereference_check((p), (sp), 1)

Given that this is a one-liner, why not just use srcu_dereference_check()
directly, with 1 as you do above to disable lockdep?

							Thanx, Paul

> +
>  /**
>   * srcu_read_lock - register a new reader for an SRCU-protected structure.
>   * @sp: srcu_struct in which to register the new reader.
> -- 
> 2.17.0.441.gb46fe60e1d-goog
>