From: Karsten Blees <[email protected]> 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 <[email protected]> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
