linux-kernel - Re: [PATCH RFC clocksource 2/5] clocksource: Retry clock read if long delays detected

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Message-ID: <c7cc57a247747d164a870ebbbedb1b8b756c73fd.camel@fb.com>
Date:   Wed, 06 Jan 2021 15:59:47 -0500
From:   Rik van Riel <riel@...com>
To:     <paulmck@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Stephen Boyd" <sboyd@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <Mark.Rutland@....com>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH RFC clocksource 2/5] clocksource: Retry clock read if
 long delays detected

On Wed, 2021-01-06 at 11:53 -0800, Paul E. McKenney wrote:
> On Wed, Jan 06, 2021 at 11:28:00AM -0500, Rik van Riel wrote:
> 
> > +               wdagain_nsec = clocksource_cyc2ns(delta, watchdog-
> > > mult, watchdog->shift);
> > +               if (wdagain_nsec < 0 || wdagain_nsec >
> > WATCHDOG_MAX_SKEW) {
> > +                       wderr_nsec = wdagain_nsec;
> > +                       if (nretries++ < max_read_retries)
> > +                               goto retry;
> > +               }
> > 
> > Given that clocksource_cyc2ns uses unsigned multiplication
> > followed by a right shift, do we need to test for <0?
> 
> I am worried about the possibility of the "shift" argument to
> clocksource_cyc2ns() being zero.  For example, unless I am missing
> something, clocksource_tsc has a zero .shift field.

Oh, good point!