Messages in this thread | | | Date | Tue, 19 Aug 2008 20:47:52 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] fix adjtimex() freq return error |
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On Tue, 19 Aug 2008 19:29:06 -0700 john stultz <johnstul@us.ibm.com> wrote:
> Martin Ziegler reported a bug in adjtimex(), where odd positive > frequencies are reduced by 1 while negative values are correct. > > See http://bugzilla.kernel.org/show_bug.cgi?id=11370 for details > > The issue was introduced in the following commit that increased > time_freq to a 64bit value and increased its resolution: > > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=074b3b87941c99bc0ce35385b5817924b1ed0c23;hp=eea83d896e318bda54be2d2770d2c5d6668d11db > > The problem is how we convert from our internal high res value back to > userland. Instead of dividing, we shift, then mult, then shift again, > causing the slight error seen in the bug. > > I don't think adjtimex() is a terribly hot path, so this patch simply > uses div_s64() to convert back to the external resolution and removes > the unused shift defines to resolve the issue. > > Signed-off-by: John Stultz <johnstul@us.ibm.com> > > Index: 2.6-git/kernel/time/ntp.c > =================================================================== > --- 2.6-git.orig/kernel/time/ntp.c > +++ 2.6-git/kernel/time/ntp.c > @@ -402,9 +402,7 @@ int do_adjtimex(struct timex *txc) > if (!(time_status & STA_NANO)) > txc->offset /= NSEC_PER_USEC; > } > - txc->freq = shift_right((s32)(time_freq >> PPM_SCALE_INV_SHIFT) * > - (s64)PPM_SCALE_INV, > - NTP_SCALE_SHIFT); > + txc->freq = div_s64(time_freq, PPM_SCALE); > txc->maxerror = time_maxerror; > txc->esterror = time_esterror; > txc->status = time_status; > Index: 2.6-git/include/linux/timex.h > =================================================================== > --- 2.6-git.orig/include/linux/timex.h > +++ 2.6-git/include/linux/timex.h > @@ -82,9 +82,6 @@ > */ > #define SHIFT_USEC 16 /* frequency offset scale (shift) */ > #define PPM_SCALE (NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC)) > -#define PPM_SCALE_INV_SHIFT 20 > -#define PPM_SCALE_INV ((1ll << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \ > - PPM_SCALE + 1) > > #define MAXPHASE 500000000l /* max phase error (ns) */ > #define MAXFREQ 500000 /* max frequency error (ns/s) */ >
Thanks, John. And Martin for the perfect bug report.
Based on Martin's observation that the bug has been there since 2.6.21 I marked this as needed in 2.6.25.x and in 2.6.26.x. However the patch doesn't apply at all to 2.6.25, so someone(tm) will have a bit of rework to do if we want to fix 2.6.25.x.
This fix caused Roman's ntp-fix-adj_offset_ss_read-bug-and-do_adjtimex-cleanup.patch (which I have queued for when Thomas wakes up) to toss a reject, but fixing that seemed pretty simple. I think. The scripts/kconfig/conf segfaults are being a bit of a PITA at present.
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