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Date: Tue, 14 Jun 2022 16:49:00 -0400
From: Rich Felker <dalias@...c.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: musl@...ts.openwall.com
Subject: Re: Question about musl's time() implementation in time.c
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@...c.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
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