linux-kernel - Re: [PATCH V6 02/16] rv: Add runtime reactors interface

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Date:   Wed, 20 Jul 2022 13:02:09 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Gabriele Paoloni <gpaoloni@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Tao Zhou <tao.zhou@...ux.dev>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-trace-devel@...r.kernel.org
Subject: Re: [PATCH V6 02/16] rv: Add runtime reactors interface

On Wed, 20 Jul 2022 18:50:39 +0200
Daniel Bristot de Oliveira <bristot@...nel.org> wrote:

> On 7/20/22 18:41, Steven Rostedt wrote:
> > On Tue, 19 Jul 2022 19:27:07 +0200
> > Daniel Bristot de Oliveira <bristot@...nel.org> wrote:
> >   
> >> +/*
> >> + * reacting_on interface.
> >> + */
> >> +static ssize_t reacting_on_read_data(struct file *filp,
> >> +				     char __user *user_buf,
> >> +				     size_t count, loff_t *ppos)
> >> +{
> >> +	char *buff;
> >> +
> >> +	mutex_lock(&rv_interface_lock);
> >> +	buff = reacting_on ? "1\n" : "0\n";
> >> +	mutex_unlock(&rv_interface_lock);  
> > Again, no need for the locks, but perhaps just to keep things sane:
> > 
> > 	buf = READ_ONCE(reacting_on) ? "1\n" : "0\n";  
> 
> So, for all files that only read/write a single variable, use READ_ONCE/WRITE_ONCE without
> locks? (and in all usage of that variable too).

Only if there's no races.

That is, taking the locks here provide no benefit over a READ_ONCE().

If there was some logic that checks if the value is still valid or not,
then that would be a different story.

For example:

static int enable_monitor(struct rv_monitor_def *mdef)
{
        int retval;

        if (!mdef->monitor->enabled) {
                retval = mdef->monitor->enable();
                if (retval)
                        return retval;
        }

        mdef->monitor->enabled = 1;

        return 0;
}

That has logic that looks to require a lock to protect things from changing
from underneath.

-- Steve