linux-kernel - Re: [PATCH v2] timekeeping: move multigrain timestamp floor handling into timekeeper

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Message-ID: <CANDhNCoLCbAYRtA+kjVNiTeNZg6UtCycNpNievLboXbjNU-a9g@mail.gmail.com>
Date: Thu, 12 Sep 2024 13:33:32 -0700
From: John Stultz <jstultz@...gle.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Thomas Gleixner <tglx@...utronix.de>, Stephen Boyd <sboyd@...nel.org>, Arnd Bergmann <arnd@...nel.org>, 
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH v2] timekeeping: move multigrain timestamp floor handling
 into timekeeper

On Thu, Sep 12, 2024 at 1:18 PM Jeff Layton <jlayton@...nel.org> wrote:
> On Thu, 2024-09-12 at 13:11 -0700, John Stultz wrote:
> > On Thu, Sep 12, 2024 at 11:02 AM Jeff Layton <jlayton@...nel.org> wrote:
> > But instead, it seems like if something has happened since the cookie
> > value was saved (another cpu getting a fine grained timestamp), your
> > ktime_get_real_ts64_mg() will fall back to returning the same coarse
> > grained time saved to the cookie, as if no time had past?
> >
> > It seems like that could cause problems:
> >
> > cpu1                                     cpu2
> > --------------------------------------------------------------------------
> >                                          t2a = ktime_get_coarse_real_ts64_mg
> > t1a = ktime_get_coarse_real_ts64_mg()
> > t1b = ktime_get_real_ts64_mg(t1a)
> >
> >                                          t2b = ktime_get_real_ts64_mg(t2a)
> >
> > Where t2b will seem to be before t1b, even though it happened afterwards.
> >
>
> Ahh no, the subtle thing about atomic64_try_cmpxchg is that it
> overwrites "old" with the value that was currently there in the event
> that the cmp fails.

Ah, ok. Thank you for the explanation there!

> So, the try_cmpxchg there will either swap the new value into place, or
> if it was updated in the meantime, "old" will now refer to the value
> that's currently in the floor word. Either is fine in this case, so we
> don't need to retry anything.


Though if cpu2 then made another call to
ktime_get_coarse_real_ts64_mg(), the value returned there will be the
same as t1b? and would be before t2b?

thanks
-john