Re: [PATCHv3] lib/ratelimit: Lockless ratelimiting

[Dmitry Safonov <dima@arista.com>]

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