linux-kernel - Re: [PATCHv3] lib/ratelimit: Lockless ratelimiting

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Date:   Thu, 02 Aug 2018 16:19:15 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, David Airlie <airlied@...ux.ie>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Theodore Ts'o <tytso@....edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCHv3] lib/ratelimit: Lockless ratelimiting

Hi Steven,
Thanks for your reply,

On Wed, 2018-08-01 at 21:48 -0400, Steven Rostedt wrote:
> I'm just catching up from my vacation. What about making rs->missed
> into an atomic, and have:
> 
> 	if (!raw_spin_trylock_irqsave(&rs->lock, flags)) {
> 		atomic_inc(&rs->missed);
> 		return 0;
> 	}
> 
> ?

Uhm. Do you mean as a preparation patch to split this on two patches?
Because it will not solve the issue where one CPU has taken rs->lock,
and is updating rs->printed, checking burst and whatnot; while the
second CPU will loose the message which was even *under* burst limit.

I.e.: there are enough of printk_ratelimit() users in tree and a
message from one can be suppressed, while shouldn't.

> You would also need to do:
> 
> 	if (time_is_before_jiffies(rs->begin + rs->interval)) {
> 		int missed = atomic_xchg(&rs->missed, 0);
> 		if (missed) {
> 
> So that you don't have a race between checking rs->missed and setting
> it
> to zero.

-- 
Thanks,
             Dmitry