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

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Message-ID: <Pine.LNX.4.64.1405151949160.911@digraph.polyomino.org.uk>
Date:	Thu, 15 May 2014 20:10:05 +0000
From:	"Joseph S. Myers" <joseph@...esourcery.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Chung-Lin Tang <cltang@...esourcery.com>,
	John Stultz <john.stultz@...aro.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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>
Subject: Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

On Thu, 15 May 2014, Arnd Bergmann wrote:

> Earlier in the thread there seemed to be a rough consensus that
> _TIME_BITS=64 wouldn't be a good idea because we wouldn't get everything
> changed to use it. For _FILE_OFFSET_BITS=64 that's ok because most
> user space doesn't ever want to deal with large files.

Well, I'm coming into this in the middle since it isn't on linux-api and 
noone has tried to work out on libc-alpha what things should look like 
from the glibc side.  _TIME_BITS seemed to make sense when I thought about 
this previously, however.

> Can you elaborate on how the switch to the new default would work?

At some appropriate release (probably after _TIME_BITS=64 is widely used 
in distributions), the glibc headers would change so that _TIME_BITS=64 is 
the default and _TIME_BITS=32 can be set to get the old interfaces.  At 
some later point _TIME_BITS=32 API support might be removed, leaving the 
old symbols as compat symbols for existing binaries.

> If it's easy, why hasn't it been done for _FILE_OFFSET_BITS already
> and what's stopping us from changing the default as soon as the interfaces
> are there? If it's hard, what would need to happen before the default
> time_t can be set?

The distribution side of the change for _FILE_OFFSET_BITS (i.e., moving to 
building libraries that way so a glibc change to the default wouldn't 
cause issues for other libraries' ABIs) has gradually been done.  The 
discussion in March on libc-alpha about changing the default tailed off.  
This is something that needs someone to take the lead with a *careful and 
detailed analysis of the information from the previous discussion* in 
order to present a properly reasoned proposal for a change to the default 
- not scattergun patches, not patches with brief or no analysis of the 
environment in which glibc is used, not dismissing concerns, but a 
properly reasoned argument for why the change should be made, along with 
details of how distributions can determine whether ABI issues would arise 
from rebuilding a particular library against newer glibc.

> > Obviously 64-bit time_t syscalls would be an appropriately narrow set of 
> > syscalls like those in the generic ABI (so glibc would implement stat for 
> > _TIME_BITS=64 using fstatat64_time64 or whatever the syscall is called, 
> > just as the stat functions for generic ABI architectures are implemented 
> > with newfstatat / fstatat64 rather than lots of separate syscalls.
> 
> This assumes that we'd leave the kernel time_t/timespec/timeval using 'long'
> and introduce a new timespec64 using a signed 64-bit type, rather than
> changing the kernel headers to the new syscalls and data structures with
> new names for the existing ones, right?

Yes.  I consider it simply common sense that new kernel headers should 
continue to work with much older glibc, meaning that the API (syscall 
names etc.) presented by the headers from headers_install should not 
change incompatibly.

(64-bit type only for time_t, of course.  There's no need for a 64-bit 
type for nanoseconds and tv_nsec is explicitly "long" in POSIX, meaning 
that if the kernel uses a 64-bit type for nanoseconds on systems where 
"long" is 32-bit in userspace, either it needs to treat the high word as 
padding or glibc needs to wrap all interfaces passing a struct timespec 
into the kernel so they clear the padding field.  There's even less need 
for a 64-bit type for microseconds.)

-- 
Joseph S. Myers
joseph@...esourcery.com
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