LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Karsten Blees <karsten.blees@gmail.com>
To: linux-kernel@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>,
	John Stultz <john.stultz@linaro.org>
Subject: [PATCH] time.c::timespec_trunc: fix nanosecond file time rounding
Date: Tue, 09 Jun 2015 19:36:13 +0200
Message-ID: <5577240D.7020309@gmail.com> (raw)

From: Karsten Blees <blees@dcon.de>
Date: Tue, 9 Jun 2015 10:50:28 +0200

The rounding optimization in timespec_trunc() is based on the incorrect
assumptions that current_kernel_time() is rounded to jiffies resolution,
and that jiffies resolution is a multiple of all potential file time
granularities.

Thus, sub-second portions of in-core file times are not rounded to on-disk
granularity. I.e. file times may change when the inode is re-read from disk
or when the file system is remounted.

File systems with on-disk resolutions of exactly 1 ns or 1 s are not
affected by this.

Steps to reproduce with e.g. UDF:

  $ dd if=/dev/zero of=udfdisk count=10000 && mkudffs udfdisk
  $ mkdir udf && mount udfdisk udf
  $ touch udf/test && stat -c %y udf/test
  2015-06-09 10:22:56.130006767 +0200
  $ umount udf && mount udfdisk udf
  $ stat -c %y udf/test
  2015-06-09 10:22:56.130006000 +0200

Remounting rounds the mtime to 1µs.

Fix the rounding in timespec_trunc() and update the documentation.

Note: This does _not_ fix the issue for FAT's 2 second mtime resolution,
as struct super_block.s_time_gran isn't prepared to handle different
ctime / mtime / atime resolutions nor resolutions > 1 second.

Signed-off-by: Karsten Blees <blees@dcon.de>
---

This issue came up in a recent discussion on the git ML about enabling
nanosecond file times on Windows, see

http://thread.gmane.org/gmane.comp.version-control.msysgit/21290/focus=21315


 kernel/time/time.c | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/kernel/time/time.c b/kernel/time/time.c
index 972e3bb..362ee06 100644
--- a/kernel/time/time.c
+++ b/kernel/time/time.c
@@ -287,23 +287,14 @@ EXPORT_SYMBOL(jiffies_to_usecs);
  * @t: Timespec
  * @gran: Granularity in ns.
  *
- * Truncate a timespec to a granularity. gran must be smaller than a second.
- * Always rounds down.
- *
- * This function should be only used for timestamps returned by
- * current_kernel_time() or CURRENT_TIME, not with do_gettimeofday() because
- * it doesn't handle the better resolution of the latter.
+ * Truncate a timespec to a granularity. gran must not be greater than a
+ * second (10^9 ns). Always rounds down.
  */
 struct timespec timespec_trunc(struct timespec t, unsigned gran)
 {
-	/*
-	 * Division is pretty slow so avoid it for common cases.
-	 * Currently current_kernel_time() never returns better than
-	 * jiffies resolution. Exploit that.
-	 */
-	if (gran <= jiffies_to_usecs(1) * 1000) {
+	if (gran <= 1) {
 		/* nothing */
-	} else if (gran == 1000000000) {
+	} else if (gran >= 1000000000) {
 		t.tv_nsec = 0;
 	} else {
 		t.tv_nsec -= t.tv_nsec % gran;
-- 
2.0.0.791.g124e248


             reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-06-09 17:36 Karsten Blees [this message]
2015-06-16 17:07 ` John Stultz
2015-06-16 22:39   ` Karsten Blees
2015-06-16 23:08     ` John Stultz
2015-06-25 12:13       ` [PATCH v2] " Karsten Blees
2015-07-01 18:07         ` John Stultz

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=5577240D.7020309@gmail.com \
    --to=karsten.blees@gmail.com \
    --cc=john.stultz@linaro.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git