| Message ID | 1416890145-30048-1-git-send-email-john.stultz@linaro.org |
|---|---|
| State | Accepted |
| Commit | cb2aa63469f81426c7406227be70b628b42f7a05 |
| Headers | show |
On 2 December 2014 at 17:32, Jeremiah Mahler <jmmahler@gmail.com> wrote: > John, > > On Mon, Nov 24, 2014 at 08:35:45PM -0800, John Stultz wrote: >> In commit 6067dc5a8c2b ("time: Avoid possible NTP adjustment mult >> overflow") a new check was added to watch for adjustments that could >> cause a mult overflow. >> >> Unfortunately the check compares a signed with unsigned value and >> ignored the case where the adjustment was negative, which causes >> spurious warn-ons on some systems (and seems like it would result in >> problematic time adjustments there as well, due to the early >> return). >> >> Thus this patch adds a check to make sure the adjustment is positive >> before we check for an overflow, and resovles the issue in my >> testing. >> >> Cc: pang.xunlei <pang.xunlei@linaro.org> >> Cc: Fengguang Wu <fengguang.wu@intel.com> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: Ingo Molnar <mingo@kernel.org> >> Reported-by: Fengguang Wu <fengguang.wu@intel.com> >> Debugged-by: pang.xunlei <pang.xunlei@linaro.org> >> Signed-off-by: John Stultz <john.stultz@linaro.org> >> --- >> kernel/time/timekeeping.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c >> index 29a7d67..2dc0646 100644 >> --- a/kernel/time/timekeeping.c >> +++ b/kernel/time/timekeeping.c >> @@ -1330,7 +1330,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk, >> * >> * XXX - TODO: Doc ntp_error calculation. >> */ >> - if (tk->tkr.mult + mult_adj < mult_adj) { >> + if ((mult_adj > 0) && (tk->tkr.mult + mult_adj < mult_adj)) { >> /* NTP adjustment caused clocksource mult overflow */ >> WARN_ON_ONCE(1); >> return; > > This change does quiet the warning but I think it does so for the wrong > reason. > > mult_adj is a signed number and tk->tkr.mult is an unsigned number. > Adding the check that (mult_adj > 0) limits the test to only positive > numbers. A positive number plus a positive number will never be less > than either of the two positive numbers. The test is always false. Hi Jeremiah, The result of a positive number plus a positive number may overflow, for example, u32 (0xFFFF_FFF0 + 0x20) will be 0x10, that's what this patch is dealing with. Thanks, Xunlei > > -- > - Jeremiah Mahler -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 29a7d67..2dc0646 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -1330,7 +1330,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk, * * XXX - TODO: Doc ntp_error calculation. */ - if (tk->tkr.mult + mult_adj < mult_adj) { + if ((mult_adj > 0) && (tk->tkr.mult + mult_adj < mult_adj)) { /* NTP adjustment caused clocksource mult overflow */ WARN_ON_ONCE(1); return;
In commit 6067dc5a8c2b ("time: Avoid possible NTP adjustment mult overflow") a new check was added to watch for adjustments that could cause a mult overflow. Unfortunately the check compares a signed with unsigned value and ignored the case where the adjustment was negative, which causes spurious warn-ons on some systems (and seems like it would result in problematic time adjustments there as well, due to the early return). Thus this patch adds a check to make sure the adjustment is positive before we check for an overflow, and resovles the issue in my testing. Cc: pang.xunlei <pang.xunlei@linaro.org> Cc: Fengguang Wu <fengguang.wu@intel.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Reported-by: Fengguang Wu <fengguang.wu@intel.com> Debugged-by: pang.xunlei <pang.xunlei@linaro.org> Signed-off-by: John Stultz <john.stultz@linaro.org> --- kernel/time/timekeeping.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)