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From: "Theodore Ts'o" <tytso@mit.edu>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Chuck Lever <chuck.lever@oracle.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Nicolas Pitre <nicolas.pitre@linaro.org>,
	Dave Chinner <david@fromorbit.com>,
	LKML Kernel <linux-kernel@vger.kernel.org>,
	linux-arch@vger.kernel.org, joseph@codesourcery.com,
	john.stultz@linaro.org, Christoph Hellwig <hch@infradead.org>,
	tglx@linutronix.de, geert@linux-m68k.org, lftan@altera.com,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	xfs@oss.sgi.com,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [RFC 11/32] xfs: convert to struct inode_time
Date: Mon, 2 Jun 2014 18:29:54 -0400
Message-ID: <20140602222954.GA29690@thunk.org> (raw)
In-Reply-To: <538CB085.5000502@zytor.com>

On Mon, Jun 02, 2014 at 10:12:37AM -0700, H. Peter Anvin wrote:
> > It would be problematic for time(2) or gettimeofday(2) to return
> > TIME_UNDEFINED, since there are programs that care about time ticking
> > forward, but I could imagine a new interface which would be permitted
> > to return a flag indicating that we don't know the current time
> > (because the CMOS battery had run down, etc.) so instead we're going
> > to be counting the number of seconds since the system was booted.
> 
> This assumes that we actually know that that is the case, which may be
> an aggressive assumption.

We won't know if the RTC clock is wrong, true --- but the kernel will
know if (a) the hardware doesn't have RTC clock at all, or if (b) the
RTC clock is ticking some time that can't be encoded using the current
time_t type.  So in that case, the fallback would be to be for the
kernel to tick starting with time_t == 0 when the system is initially
booted, and the "time indefinite flag" would be set.

Now assume that we have a new system call, gettimestampofday(2), which
returns a new timestamp structure which has a 64-bit ts_sec field, the
ts_nsec field (ala struct timespec), and a ts_flags field, where the
kernel could signal things like "time invalid", or "time can't be
encoded in the legacy time_t type", or "I'm not sure if the time is
correct" --- i.e., because the RTC battery isn't working.

Not all hardware might be able to support the last, of course, but if
the battery is low, or the system has been exposed to very low
temperatures (or large amounts of cosmic radiation, etc.)  the RTC
time may just be plain wrong.  No system is going to be perfect, but
it should be possible to make htings better, at for certain classes of
hardware.

And since we are already returning (time_t) -1 in some cases, we might
as well try to make things a bit more formal.

					- Ted


  parent reply index

Thread overview: 124+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-05-30 20:01 [RFC 00/32] making inode time stamps y2038 ready Arnd Bergmann
2014-05-30 20:01 ` [RFC 01/32] fs: introduce new 'struct inode_time' Arnd Bergmann
2014-05-31  7:56   ` Geert Uytterhoeven
2014-05-31  8:39     ` Andreas Schwab
2014-05-31 13:19       ` Geert Uytterhoeven
2014-05-31 13:46         ` Andreas Schwab
2014-05-31 14:54       ` Arnd Bergmann
2014-05-31 16:15         ` Geert Uytterhoeven
2014-05-31  9:03   ` H. Peter Anvin
2014-05-31 14:53     ` Arnd Bergmann
2014-05-31 14:55       ` H. Peter Anvin
2014-05-30 20:01 ` [RFC 02/32] uapi: add struct __kernel_timespec{32,64} Arnd Bergmann
2014-05-30 20:18   ` H. Peter Anvin
2014-05-31 15:09     ` Arnd Bergmann
2014-05-30 20:01 ` [RFC 03/32] fs: introduce sys_utimens64at Arnd Bergmann
2014-05-31  9:22   ` Andreas Schwab
2014-05-31 14:55     ` Arnd Bergmann
2014-05-30 20:01 ` [RFC 04/32] fs: introduce sys_newfstat64/sys_newfstatat64 Arnd Bergmann
2014-05-30 20:01 ` [RFC 05/32] arch: hook up new stat and utimes syscalls Arnd Bergmann
2014-05-30 20:01 ` [RFC 06/32] isofs: fix timestamps beyond 2027 Arnd Bergmann
2014-05-31  7:59   ` Geert Uytterhoeven
2014-05-31  8:47     ` H. Peter Anvin
2014-05-30 20:01 ` [RFC 07/32] fs/nfs: convert to struct inode_time Arnd Bergmann
2014-05-30 20:01 ` [RFC 08/32] fs/ceph: convert to 'struct inode_time' Arnd Bergmann
2014-05-30 20:01 ` [RFC 09/32] fs/pstore: convert to struct inode_time Arnd Bergmann
2014-05-30 21:14   ` Kees Cook
2014-05-30 20:01 ` [RFC 10/32] fs/coda: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 11/32] xfs: " Arnd Bergmann
2014-05-31  0:37   ` Dave Chinner
2014-05-31  0:41     ` H. Peter Anvin
2014-05-31  1:14       ` Dave Chinner
2014-05-31  1:22         ` H. Peter Anvin
2014-05-31  5:54           ` Dave Chinner
2014-05-31  8:41             ` H. Peter Anvin
2014-05-31 15:46               ` Nicolas Pitre
2014-06-01 19:56                 ` Arnd Bergmann
2014-06-01 20:26                   ` H. Peter Anvin
2014-06-02 11:02                     ` Arnd Bergmann
2014-06-02  1:36                   ` Nicolas Pitre
2014-06-02  2:22                     ` Dave Chinner
2014-06-02  7:09                       ` Geert Uytterhoeven
2014-06-02 10:56                     ` Arnd Bergmann
2014-06-02 11:57                       ` Theodore Ts'o
2014-06-02 12:38                         ` Arnd Bergmann
2014-06-02 13:15                           ` Theodore Ts'o
2014-06-02 12:52                         ` Arnd Bergmann
2014-06-02 13:07                           ` Theodore Ts'o
2014-06-02 15:01                             ` Arnd Bergmann
2014-06-02 14:52                         ` H. Peter Anvin
2014-06-02 15:04                       ` Chuck Lever
2014-06-02 15:31                         ` Theodore Ts'o
2014-06-02 17:12                           ` H. Peter Anvin
2014-06-02 18:50                             ` Arnd Bergmann
2014-06-02 22:29                             ` Theodore Ts'o [this message]
2014-06-02 22:32                               ` H. Peter Anvin
2014-06-02 23:32                                 ` Theodore Ts'o
2014-06-02 23:33                                   ` H. Peter Anvin
2014-06-03 13:09                                   ` Roger Willcocks
2014-06-02 18:52                         ` Arnd Bergmann
2014-06-02 18:58                         ` Roger Willcocks
2014-06-02 19:04                           ` Chuck Lever
2014-06-02 19:10                             ` Arnd Bergmann
2014-06-01  0:39               ` Dave Chinner
2014-06-02 14:00             ` Joseph S. Myers
2014-05-31 15:37         ` Arnd Bergmann
2014-06-01  0:24           ` Dave Chinner
2014-06-02  0:28             ` Dave Chinner
2014-06-02 11:35               ` Roger Willcocks
2014-06-02 11:43               ` Arnd Bergmann
2014-06-03  0:32                 ` Dave Chinner
2014-06-03  7:33                   ` Arnd Bergmann
2014-06-03  8:41                     ` Dave Chinner
2014-06-03  9:16                       ` Arnd Bergmann
2014-05-30 20:01 ` [RFC 12/32] btrfs: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 13/32] ext3: " Arnd Bergmann
2014-05-31  9:10   ` H. Peter Anvin
2014-05-31 14:32     ` Arnd Bergmann
2014-05-30 20:01 ` [RFC 14/32] ext4: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 15/32] cifs: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 16/32] ntfs: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 17/32] ubifs: " Arnd Bergmann
2014-06-02  7:54   ` Artem Bityutskiy
2014-05-30 20:01 ` [RFC 18/32] ocfs2: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 19/32] fs/fat: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 20/32] afs: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 21/32] udf: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 22/32] fs: convert simple fs to inode_time Arnd Bergmann
2014-05-30 23:06   ` Greg Kroah-Hartman
2014-05-30 20:01 ` [RFC 23/32] logfs: convert to struct inode_time Arnd Bergmann
2014-05-30 20:01 ` [RFC 24/32] hfs, hfsplus: " Arnd Bergmann
2014-05-31 14:23   ` Vyacheslav Dubeyko
2014-05-30 20:01 ` [RFC 25/32] gfs2: " Arnd Bergmann
2014-06-02  9:52   ` Steven Whitehouse
2014-05-30 20:01 ` [RFC 26/32] reiserfs: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 27/32] jffs2: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 28/32] adfs: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 29/32] f2fs: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 30/32] fuse: " Arnd Bergmann
2014-05-30 20:01 ` [RFC 31/32] scsi: fnic: use current_kernel_time() for timestamp Arnd Bergmann
2014-05-30 20:01 ` [RFC 32/32] fs: use new inode_time definition unconditionally Arnd Bergmann
2014-05-31 14:30 ` [RFC 00/32] making inode time stamps y2038 ready Vyacheslav Dubeyko
2014-06-03 12:21   ` Arnd Bergmann
2014-05-31 14:51 ` Richard Cochran
2014-05-31 15:23   ` Arnd Bergmann
2014-05-31 18:22     ` Richard Cochran
2014-05-31 19:34       ` H. Peter Anvin
2014-06-01  4:46         ` Richard Cochran
2014-06-01  4:44     ` Richard Cochran
2014-06-02 13:52 ` Joseph S. Myers
2014-06-02 19:19   ` Arnd Bergmann
2014-06-02 19:26     ` H. Peter Anvin
2014-06-02 19:55       ` Arnd Bergmann
2014-06-02 21:57         ` H. Peter Anvin
2014-06-03 14:22           ` Arnd Bergmann
2014-06-03 14:33             ` Joseph S. Myers
2014-06-03 14:37               ` Arnd Bergmann
2014-06-03 21:38             ` Dave Chinner
2014-06-04 15:03               ` Arnd Bergmann
2014-06-04 17:30                 ` Nicolas Pitre
2014-06-04 19:24                   ` Arnd Bergmann
2014-06-05  0:10                     ` H. Peter Anvin
2014-06-10  9:54                       ` Arnd Bergmann
2014-06-02 21:02     ` Joseph S. Myers
2014-06-04 15:05       ` Arnd Bergmann

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