Re: [PATCH] use a stable clock reference in vdso vgetns

[john stultz <johnstul@us.ibm.com>]

On Wed, 2010-10-13 at 23:44 -0300, Glauber Costa wrote:
> On Wed, Oct 13, 2010 at 06:36:55PM -0700, john stultz wrote:
> > On Wed, 2010-10-13 at 11:07 -0300, Glauber Costa wrote:
> > > 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?
> > 
> > Errr.. Sorry, are you waiting for me to implement this? Or did you want
> > a comment on your earlier mail? (Apologies, I've been a bit scattered
> > here).
> > 
> np. Comments on the earlier e-mail.
> I tried a bit more with that, and sent two other e-mails arguing why I thought
> that was the right approach. (well, the second best, just after unification)
> 
> > I think trying to unify the two implementations would be nice if you're
> > up to trying, but if not, go ahead and push your patch and that will fix
> > the bug until I can get around to looking at the unification.
> Unfortunately, I have other things under my radar now, so if people
> don't see a problem, I'd like to have this particular problem solved.
> The patch seems right anyway.

Ok. Then go ahead and push it in and I'll try to get the unification
done later.

thanks
-john