Re: PROBLEM: monotonic clock going backwards on ppc64

On Sat, 2 Mar 2019, Michael Ellerman wrote:

quoted

The problem seems to be in vDSO code in arch/powerpc/kernel/vdso64/gettimeofday.S.

You're right, the wall-to-monotonic offset (wtom_clock_sec) is a signed
32-bit value, so that seems like it's going to have problems.

If I do `date -s 2037-1-1` I see:

[   26.024061] update_vsyscall: tk->wall_to_monotonic.tv_sec -2114341175
[   26.042633] update_vsyscall: vdso_data->wtom_clock_sec    -2114341175

Which looks sane.

But then 2040-1-1 shows:

[   32.617020] update_vsyscall: tk->wall_to_monotonic.tv_sec -2208949168
[   32.632642] update_vsyscall: vdso_data->wtom_clock_sec     2086018128

ie. the larger negative offset has overflowed and become positive.

But then when we go back to 2037 we get a negative offset again and
monotonic time appears to go backward and things are unhappy.

I don't know this code well, but the patch below *appears* to work. I'll
have a closer look on Monday.

Looks about right. The core hands in the 64bit offset already.

Thanks,

	tglx