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authorKarsten Blees <karsten.blees@gmail.com>2015-06-25 14:13:55 +0200
committerJohn Stultz <john.stultz@linaro.org>2015-08-17 11:23:46 -0700
commitde4a95faf173be1e798c37ca486dfcb234a0941b (patch)
tree7bf9d479641bb7d09b44b54c0771643af19a54fd /kernel/time
parent38bf985b05625df3fbbc1dbf543bdd2da447c2af (diff)
time: Fix nanosecond file time rounding in timespec_trunc()
timespec_trunc() avoids rounding if granularity <= nanoseconds-per-jiffie (or TICK_NSEC). This optimization assumes that: 1. current_kernel_time().tv_nsec is already rounded to TICK_NSEC (i.e. with HZ=1000 you'd get 1000000, 2000000, 3000000... but never 1000001). This is no longer true (probably since hrtimers introduced in 2.6.16). 2. TICK_NSEC is evenly divisible by all possible granularities. This may be true for HZ=100, 250, 1000, but obviously not for HZ=300 / TICK_NSEC=3333333 (introduced in 2.6.20). 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. This affects all file systems with file time granularities > 1 ns and < 1s, e.g. CEPH (1000 ns), UDF (1000 ns), CIFS (100 ns), NTFS (100 ns) and FUSE (configurable from user mode via struct fuse_init_out.time_gran). 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 truncates the mtime to 1 µs. Fix the rounding in timespec_trunc() and update the documentation. timespec_trunc() is exclusively used to calculate inode's [acm]time (mostly via current_fs_time()), and always with super_block.s_time_gran as second argument. So this can safely be changed without side effects. Note: This does _not_ fix the issue for FAT's 2 second mtime resolution, as super_block.s_time_gran isn't prepared to handle different ctime / mtime / atime resolutions nor resolutions > 1 second. Cc: Prarit Bhargava <prarit@redhat.com> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Karsten Blees <blees@dcon.de> Signed-off-by: John Stultz <john.stultz@linaro.org>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/time.c22
1 files changed, 8 insertions, 14 deletions
diff --git a/kernel/time/time.c b/kernel/time/time.c
index 85d5bb1d67eb..34dbd4209e4a 100644
--- a/kernel/time/time.c
+++ b/kernel/time/time.c
@@ -287,26 +287,20 @@ 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. Always rounds down. gran must
+ * not be 0 nor greater than a second (NSEC_PER_SEC, or 10^9 ns).
*/
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) {
+ /* Avoid division in the common cases 1 ns and 1 s. */
+ if (gran == 1) {
/* nothing */
- } else if (gran == 1000000000) {
+ } else if (gran == NSEC_PER_SEC) {
t.tv_nsec = 0;
- } else {
+ } else if (gran > 1 && gran < NSEC_PER_SEC) {
t.tv_nsec -= t.tv_nsec % gran;
+ } else {
+ WARN(1, "illegal file time granularity: %u", gran);
}
return t;
}