Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 14 Mar 2012 18:46:29 -0700 | Subject | Re: [PATCH 1/2] time: x86: Fix race switching from vsyscall to non-vsyscall clock |
| |
On Wed, Mar 14, 2012 at 6:43 PM, Andy Lutomirski <luto@amacapital.net> wrote: > On Wed, Mar 14, 2012 at 5:42 PM, John Stultz <john.stultz@linaro.org> wrote: >> On 03/14/2012 05:34 PM, Thomas Gleixner wrote: >>> >>> On Wed, 14 Mar 2012, John Stultz wrote: >>>> >>>> notrace static noinline int do_realtime(struct timespec *ts) >>>> { >>>> unsigned long seq, ns; >>>> + int mode; >>> >>> Please keep a newline between declarations and code. >> >> >> Fixed below. Thanks! >> (Let me know if you see whitespace damage, I switched mail clients today and >> am learning the quirks here.) >> -john >> >> >> >> When switching from a vsyscall capable to a non-vsyscall capable >> clocksource, there was a small race, where the last vsyscall >> gettimeofday before the switch might return a invalid time value >> using the new non-vsyscall enabled clocksource values after the >> switch is complete. >> >> This is due to the vsyscall code checking the vclock_mode once >> outside of the seqcount protected section. After it reads the >> vclock mode, it doesn't re-check that the sampled clock data >> that is obtained in the seqcount critical section still matches. >> >> The fix is to sample vclock_mode inside the protected section, >> and as long as it isn't VCLOCK_NONE, return the calculated >> value. If it has changed and is now VCLOCK_NONE, fall back >> to the syscall gettime calculation. >> >> v2: >> * Cleanup checks as suggested by tglx >> * Also fix same issue present in gettimeofday path >> >> CC: Andy Lutomirski<luto@amacapital.net> >> CC: Thomas Gleixner<tglx@linutronix.de> >> Signed-off-by: John Stultz<john.stultz@linaro.org> >> --- >> arch/x86/vdso/vclock_gettime.c | 68 >> +++++++++++++++++++++++++-------------- >> 1 files changed, 43 insertions(+), 25 deletions(-) >> > > Looks reasonable to me. I like this approach better than the earlier > way -- it's likely to cause less slowdown in the VCLOCK_TSC case. > > That being said, I think you might have a bug: > > notrace static inline long vgetns(void) > { > long v; > cycles_t cycles; > if (gtod->clock.vclock_mode == VCLOCK_TSC) > cycles = vread_tsc(); > else > cycles = vread_hpet(); > v = (cycles - gtod->clock.cycle_last) & gtod->clock.mask; > return (v * gtod->clock.mult) >> gtod->clock.shift; > } > > In the VCLOCK_NONE, you'll access the hpet mapping. But in > hpet_enable, hpet_set_mapping isn't called and this will crash, I > think. >
One way to fix it would be to unconditionally map the page. Then there's no performance loss for the tsc case.
--Andy -- 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/
| |