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

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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

#93785 — Bug#1144613: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency

Here's a simple program to reproduce the issue. On my machine,
a difference is output many times every second (apparently,
when the clock is very close to an integer number of seconds).

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/stat.h>

const char fname[] = "timetest.txt";

int main(void)
{
  while (1)
    {
      struct stat st;
      FILE *f;
      time_t t;

      f = fopen(fname, "w");
      if (f == NULL)
        exit(1);
      fclose(f);
      t = time(NULL);
      if (stat(fname, &st) != 0)
        exit(1);
      if (t < st.st_mtime)
        printf("%ld %ld\n", (long) t, (long) st.st_mtime);
    }
}

-- 
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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#93787

Control: retitle -1 linux-image-7.1.6+deb14-amd64: st_mtime and time() inconsistency

On 2026-08-19 13:48:48 +0200, Vincent Lefevre wrote:
> Here's a simple program to reproduce the issue. On my machine,
> a difference is output many times every second (apparently,
> when the clock is very close to an integer number of seconds).
[...]

This also occurs in /dev/shm (tmpfs).

And I cannot reproduce this issue under Debian 13 (trixie) with
the 6.12.96+deb13-amd64 kernel.

-- 
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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#93788 — Processed: Re: linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency

Processing control commands:

> retitle -1 linux-image-7.1.6+deb14-amd64: st_mtime and time() inconsistency
Bug #1144613 [src:linux] linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency
Changed Bug title to 'linux-image-7.1.6+deb14-amd64: st_mtime and time() inconsistency' from 'linux-image-7.1.6+deb14-amd64: ext4 st_mtime and time() inconsistency'.

-- 
1144613: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1144613
Debian Bug Tracking System
Contact owner@bugs.debian.org with problems

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#93794

Hi,

On Wed, Aug 19, 2026 at 02:28:51PM +0200, Vincent Lefevre wrote:
> Control: retitle -1 linux-image-7.1.6+deb14-amd64: st_mtime and time() inconsistency
> 
> On 2026-08-19 13:48:48 +0200, Vincent Lefevre wrote:
> > Here's a simple program to reproduce the issue. On my machine,
> > a difference is output many times every second (apparently,
> > when the clock is very close to an integer number of seconds).
> [...]
> 
> This also occurs in /dev/shm (tmpfs).
> 
> And I cannot reproduce this issue under Debian 13 (trixie) with
> the 6.12.96+deb13-amd64 kernel.

I think this is a consequence of 4e40eff0b5737c ("fs: add
infrastructure for multigrain timestamps") and following in 6.13-rc1.

time(2) has only an accurancy in second, so the code might need to
use clock_gettime(2).

Regards,
Salvatore

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#93796

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.
> 
> 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).

-- 
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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#93798

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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#93801

On 2026-08-19 17:00:45 +0200, Vincent Lefevre wrote:
> 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).

With the 6.12.96+deb13-amd64 kernel, I get the inconsistency
between time(NULL) and clock_gettime(CLOCK_REALTIME, ...),
and even with 6.1.0-52-amd64 (bookworm).

This could mean that this old issue propagated to the filesystem,
probably due to 4e40eff0b5737c.

-- 
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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