linux-kernel - Re: [PATCH 00/25] Change time_t and clock

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Date:	Tue, 13 May 2014 20:24:59 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ley Foon Tan <lftan@...era.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LeyFoon Tan <lftan.linux@...il.com>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

On Tue, May 13, 2014 at 8:10 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tuesday 13 May 2014 10:46:40 Christoph Hellwig wrote:
>> On Tue, May 13, 2014 at 05:33:01PM +0200, Thomas Gleixner wrote:
>> > and it
>> > would be dumb as hell to have new archs use time_t 32bit when we are
>> > currently twisting our brain around how to solve the y2038
>> > problem. Simply because we can not do the BSD flag day approach and
>> > change it.
>>
>> I don't think it's a good idea to have minor new architectures
>> pointlessly different than the major ones.  Especially given that we'll
>> absolutely have to fir the y2038 problem for 32-bit arm and probably
>> x86 anyway.
>
> I've just spent two days looking at stuff that uses time_t inside
> of the kernel, to get a better idea of what we actually need to
> do to get provide new user interfaces for the existing architectures.
>
> My impression so far is that we're better off fixing it for the
> existing architectures first and then using the new interfaces
> exclusively on new ones, rather than changing over the ABI for
> all new architectures at this point, which would likely create
> yet another variant to maintain in the long run.

Makes sense.

> Using 64-bit time_t on x32 is fine, because it's fast to operate
> in user space with 64-bit registers, and the kernel is 64-bit
> anyway. Inside of the kernel, we may get into trouble using
> a 64-bit time_t on 32-bit architectures because of the overhead
> in 64-bit math, e.g. all the timekeeping code that is based on
> timespec or some code paths in file systems and network code where
> we actually require division of time_t values.

While going over time_t uses, have you found a pattern for use cases
involving division of time_t values in filesystem and networking code?

> We clearly have to change that code in some for to deal with y2038,
> but 64-bit time_t may not be the best option. A lot of the
> in-kernel code can probably use ktime_t, which we can change
> to a different representation (e.g. 34 bit seconds) if needed,
> and all the code that is only interested in relative time
> (e.g. nanosleep) doesn't have to change at all.

Yeah. 32-bit uptimes should be good enough for everyone (don't quote
me on that), so adding a 64-bit offset when there's a need for absolute
time should be OK.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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