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From: Rich Felker <dalias@libc.org>
To: Arnd Bergmann <arnd@kernel.org>
Cc: musl@lists.openwall.com
Subject: Re: [musl] Question about musl's time() implementation in time.c
Date: Tue, 14 Jun 2022 16:49:00 -0400	[thread overview]
Message-ID: <20220614204900.GI7074@brightrain.aerifal.cx> (raw)
In-Reply-To: <CAK8P3a0d9hOuv+RiN_eQZtmfc7DCbJe6nEhL+W1AU2kFjiDZWw@mail.gmail.com>

On Tue, Jun 14, 2022 at 10:37:25PM +0200, Arnd Bergmann wrote:
> On Tue, Jun 14, 2022 at 7:00 PM Rich Felker <dalias@libc.org> wrote:
> > On Tue, Jun 14, 2022 at 06:50:40PM +0200, Arnd Bergmann wrote:
> > > The coarse time can be up to one timer tick behind, so reading
> > > CLOCK_REALTIME first
> > > can give you the exact second with a small nanosecond value, while the
> > > utime will still
> > > set the previous value.
> > >
> > > Can you change the test case to check if the later time is less than
> > > clock_getres(CLOCK_REALTIME_COARSE, ...) behind?
> >
> > This seems like a bug that the kernel uses the wrong clock for setting
> > file timestamps. It can result in seeing events out-of-order (exactly
> > as described in this thread). This should really be fixed or at least
> > made switchable so users who care can fix it.
> 
> I can't find any reference to what the correct clock is here,
> are you sure that this is specified at all? The decision to use the coarse
> time in the kernel is definitely intentional, as reading the hardware
> clocksource can be expensive (depending on the hardware), and
> changing the behavior would likely break applications that rely on
> it being the coarse clock.

POSIX specifies operations that set the file timestamps in terms of
the system (CLOCK_REALTIME) clock, not a weird implementation-defined
alternate clock.

Maybe you're right that getting the correct clock is costly on some
archs, but it's almost surely not on any arch that admits vdso
clock_gettime. And "race that causes applications to see wrong
ordering of filesystem operations with respect to other activity for
the sake of performance" does not seem like a good idea.

Rich

  reply	other threads:[~2022-06-14 20:49 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-07 12:25 Zev Levy Stevenson
2022-06-07 14:29 ` Arnd Bergmann
2022-06-07 16:30   ` Rich Felker
2022-06-10  8:52     ` Zev Levy Stevenson
2022-06-14 16:50       ` Arnd Bergmann
2022-06-14 17:00         ` Rich Felker
2022-06-14 20:37           ` Arnd Bergmann
2022-06-14 20:49             ` Rich Felker [this message]
2022-06-14 21:11               ` Arnd Bergmann
2022-06-14 23:28                 ` Rich Felker
2022-06-15 12:09                   ` Arnd Bergmann
2022-06-15 16:55                     ` Adhemerval Zanella
2022-06-16  9:06                     ` Thomas Gleixner
2022-06-16 14:57       ` Rich Felker

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