linux-kernel - Re: [PATCH 3/7] Functions to fetch POSIX dynamic clock object

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Message-ID: <87h7rdecyo.fsf@nanos.tec.linutronix.de>
Date:   Fri, 02 Oct 2020 01:35:59 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Erez Geva <erez.geva.ext@...mens.com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>, Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Michal Kubecek <mkubecek@...e.cz>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Cochran <richardcochran@...il.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Vladis Dronov <vdronov@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Vedang Patel <vedang.patel@...el.com>,
        Simon Sudler <simon.sudler@...mens.com>,
        Andreas Meisinger <andreas.meisinger@...mens.com>,
        Andreas Bucher <andreas.bucher@...mens.com>,
        Henning Schild <henning.schild@...mens.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Andreas Zirkler <andreas.zirkler@...mens.com>,
        Ermin Sakic <ermin.sakic@...mens.com>,
        An Ninh Nguyen <anninh.nguyen@...mens.com>,
        Michael Saenger <michael.saenger@...mens.com>,
        Bernd Maehringer <bernd.maehringer@...mens.com>,
        Gisela Greinert <gisela.greinert@...mens.com>,
        Erez Geva <erez.geva.ext@...mens.com>,
        Erez Geva <ErezGeva2@...il.com>
Subject: Re: [PATCH 3/7] Functions to fetch POSIX dynamic clock object

On Thu, Oct 01 2020 at 22:51, Erez Geva wrote:
> Add kernel functions to fetch a pointer to a POSIX dynamic clock
> using a user file description dynamic clock ID.

And how is that supposed to work. What are the lifetime rules?
  
> +struct posix_clock *posix_clock_get_clock(clockid_t id)
> +{
> +	int err;
> +	struct posix_clock_desc cd;

The core code uses reverse fir tree ordering of variable declaration
based on the length:

	struct posix_clock_desc cd;
        int err;

> +	/* Verify we use posix clock ID */
> +	if (!is_clockid_fd_clock(id))
> +		return ERR_PTR(-EINVAL);
> +
> +	err = get_clock_desc(id, &cd);

So this is a kernel interface and get_clock_desc() does:

	struct file *fp = fget(clockid_to_fd(id));

How is that file descriptor valid in random kernel context?

> +	if (err)
> +		return ERR_PTR(err);
> +
> +	get_device(cd.clk->dev);

The purpose of this is? Comments are overrated...

> +	put_clock_desc(&cd);
> +
> +	return cd.clk;
> +}
> +EXPORT_SYMBOL_GPL(posix_clock_get_clock);
> +
> +int posix_clock_put_clock(struct posix_clock *clk)
> +{
> +	if (IS_ERR_OR_NULL(clk))
> +		return -EINVAL;
> +	put_device(clk->dev);
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(posix_clock_put_clock);
> +
> +int posix_clock_gettime(struct posix_clock *clk, struct timespec64 *ts)
> +{
> +	int err;
> +
> +	if (IS_ERR_OR_NULL(clk))
> +		return -EINVAL;
> +
> +	down_read(&clk->rwsem);

Open coding the logic of get_posix_clock() and having a copy here and
in the next function is really useful.

Thanks,

        tglx