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Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency

Cross-posted to 2 groups.

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On 2026-08-19 16:22:53 +0200, Vincent Lefevre wrote:
> On 2026-08-19 15:50:16 +0200, Salvatore Bonaccorso wrote:
> > I think this is a consequence of 4e40eff0b5737c ("fs: add
> > infrastructure for multigrain timestamps") and following in 6.13-rc1.

Probably not (or not entirely), as there is another issue not involving
the filesystem (see below).

> > time(2) has only an accurancy in second, so the code might need to
> > use clock_gettime(2).
> 
> The accuracy does not matter. This is a question of consistency.
> If an event A occurs before an event B, one expects to have
> time(A) <= time(B).

Now, comparing time(NULL) and clock_gettime(CLOCK_REALTIME, &ts):

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void)
{
  while (1)
    {
      struct timespec ts;
      time_t t;

      clock_gettime(CLOCK_REALTIME, &ts);
      t = time(NULL);
      if (ts.tv_nsec < 0)
        exit(1);
      if (t < ts.tv_sec)
        printf("%ld %ld\n", (long) t, (long) ts.tv_sec);
    }
}

Since clock_gettime is called before time(NULL), one expects that
its time (including nanoseconds) is <= t, and since the number of
nanoseconds is non-negative (which is checked by the above code),
one should have ts.tv_sec <= t, i.e. the test t < ts.tv_sec should
be always false. But I get output at each second!

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / Pascaline project (LIP, ENS-Lyon)

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Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency Vincent Lefevre <vincent@vinc17.net> - 2026-08-19 14:00 +0200
  Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency Vincent Lefevre <vincent@vinc17.net> - 2026-08-19 14:30 +0200
    Processed: Re: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and  time() inconsistency "Debian Bug Tracking System" <owner@bugs.debian.org> - 2026-08-19 14:30 +0200
    Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency Salvatore Bonaccorso <carnil@debian.org> - 2026-08-19 16:00 +0200
      Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency Vincent Lefevre <vincent@vinc17.net> - 2026-08-19 16:30 +0200
        Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency Vincent Lefevre <vincent@vinc17.net> - 2026-08-19 17:20 +0200
          Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency Vincent Lefevre <vincent@vinc17.net> - 2026-08-19 19:20 +0200

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