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Date:   Thu, 21 Jul 2022 14:08:38 +0200
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Subject: Re: [PATCH V6 04/16] rv/include: Add deterministic automata monitor
 definition via C macros
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To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Wim Van Sebroeck <wim@linux-watchdog.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Gabriele Paoloni <gpaoloni@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Clark Williams <williams@redhat.com>,
        Tao Zhou <tao.zhou@linux.dev>,
        Randy Dunlap <rdunlap@infradead.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-trace-devel@vger.kernel.org
References: <cover.1658244826.git.bristot@kernel.org>
 <9ffc05b67fff087413143a420373731e0e34eef4.1658244826.git.bristot@kernel.org>
 <20220720160606.3e672b55@gandalf.local.home>
From:   Daniel Bristot de Oliveira <bristot@kernel.org>
In-Reply-To: <20220720160606.3e672b55@gandalf.local.home>
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On 7/20/22 22:06, Steven Rostedt wrote:
>> +/*												\
>> + * da_monitor_enabled_##name - checks if the monitor is enabled					\
>> + */												\
>> +static inline bool da_monitor_enabled_##name(void)						\
>> +{												\
> Should we add a:
> 
> 	smp_rmb();
> 
> here? And then a smp_wmb() where these switches get updated?
>

Makes sense.

Should I also add the READ_ONCE/WRITE_ONCE? like

smp_rmb()
READ_ONCE(var)

WRITE_ONCE(var, value)
smp_wmb()

for all these on/off knobs, or just the barriers?

> I guess how critical is it that these turn off immediately after the switch
> is flipped?

It is not critical to continue the execution of those that have already crossed by
the variable. Still, waiting for the tracepoints to finish their execution before
returning to the user-space task that disabled the variable might be a good thing.

IIRC, we can do that via RCU... like, synchronize_rcu()?

>> +	/* global switch */									\
>> +	if (unlikely(!rv_monitoring_on()))							\
>> +		return 0;									\
>> +												\
>> +	/* monitor enabled */									\
>> +	if (unlikely(!rv_##name.enabled))							\
>> +		return 0;									\
>> +												\
>> +	return 1;										\
>> +}												\
>> +												\