Messages in this thread | | | Date | Wed, 13 Oct 2010 11:07:46 -0300 | From | Glauber Costa <> | Subject | Re: [PATCH] use a stable clock reference in vdso vgetns |
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On Fri, Oct 01, 2010 at 10:49:53AM -0700, john stultz wrote: > On Fri, Oct 1, 2010 at 6:09 AM, Glauber Costa <glommer@redhat.com> wrote: > > When using vdso services for clock_gettime, we test for the ability > > of a fine-grained measurement through the existance of a vread() function. > > > > However, from the time we test it, to the time we use it, vread() reference > > may not be valid anymore. It happens, for example, when we change the current > > clocksource from one that provides vread (say tsc) to one that lacks it > > (say acpi_pm), in the middle of clock_gettime routine. > > > > seqlock does not really protect us, since readers here won't stop the writers > > to change references. The proposed solution is to grab a copy of the clock > > structure early, and use it as a stable reference onwards. > > Ah. Good find! The fix looks reasonable to me. However, its likely the > similar code in arch/x86/kernel/vsyscall_64.c will need a similar fix. > > Awhile back there was some motivation to merge the two vdso/vsyscall > implementations to avoid the duplication, but my memory is failing on > why that didn't happen. I feel like it had to do with complication > with the way the two implementations are mapped out to userland. Even > so, it seems a shared forced inline method would resolve the issue, so > maybe it just fell off the todo list? News here?
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