linux-kernel - Re: [RFC 00/20] ns: Introduce Time Namespace

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Date:   Mon, 1 Oct 2018 23:20:47 +0000
From:   Andrey Vagin <avagin@...tuozzo.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Dmitry Safonov <dima@...sta.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        "containers@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        "criu@...nvz.org" <criu@...nvz.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC 00/20] ns: Introduce Time Namespace

On Thu, Sep 27, 2018 at 11:41:49PM +0200, Thomas Gleixner wrote:
> On Thu, 27 Sep 2018, Thomas Gleixner wrote:
> > Add time skew via NTP/PTP into the picture and you might have to adjust
> > timers as well, because you need to guarantee that they are not expiring
> > early.
> > 
> > I haven't looked through Dimitry's patches yet, but I don't see how this
> > can work at all without introducing subtle issues all over the place.
> 
> And just a quick scan tells me that this is broken. Timers will expire
> early or late. The latter is acceptible to some extent, but larger delays
> might come with surprise. Expiring early is an absolute nono.

Do you mean that we have to adjust all timers after changing offset for
CLOCK_MONOTONIC or CLOCK_BOOTTIME? Our idea is that offsets for
monotonic and boot times will be set immediately after creating a time
namespace before using any timers.

It is interesting to think what a use-case for changing these offsets
after creating timers. It may be useful for testing needs. A user sets a
timer in an hour and then change a clock offset forward and check that a
test application handles the timer properly.

> 
> Thanks,
> 
> 	tglx
>