linux-kernel - Re: [PATCH] time, ntp: Do not update time

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Message-ID: <20150206103815.GB23998@localhost>
Date:	Fri, 6 Feb 2015 11:38:15 +0100
From:	Miroslav Lichvar <mlichvar@...hat.com>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] time, ntp: Do not update time_state in middle of leap

On Thu, Feb 05, 2015 at 08:20:08AM -0500, Prarit Bhargava wrote:
> On 02/04/2015 11:30 AM, Miroslav Lichvar wrote:
> > If after that, adjtimex() will return with TIME_ERROR as expected, or
> > not?
> 
> It is possible that an adjtimex() will set the time_state here back to TIME_OK
> and return TIME_OK to userspace.  Again, and I want to stress this, this is
> extremely unlikely to happen.  I only hit this due to a bug in a test program.
> But at the end of the day, it is possible that this happens and we should
> protect against it.

Could it break any applications? I guess PLL is normally disabled only
when a time synchronization process ends. FWIW, the reference
nanokernel implementation has this too.

> >> -	if ((time_status & STA_PLL) && !(txc->status & STA_PLL)) {
> >> +	if ((time_status & STA_PLL) && !(txc->status & STA_PLL) &&
> >> +	    (time_state != TIME_OOP)) {
> >>  		time_state = TIME_OK;
> >>  		time_status = STA_UNSYNC;
> >>  		/* restart PPS frequency calibration */
> > 
> > Shouldn't be time_status reset and the PPS calibration restarted even
> > when state is TIME_OOP?
> 
> No, this should only happen after the leap second is done IMO (which should be
> no more than 2 seconds later).

But that will not happen automatically, the application would have to
enable and disable the PLL again. Interestingly, the "time_status =
STA_UNSYNC" assignment doesn't seem to do anything here, as the
variable is always reset couple lines after that, STA_UNSYNC is not a
readonly flag.

-- 
Miroslav Lichvar
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