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

[Glauber Costa <glommer@redhat.com>]

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?

Actually vsyscall updates seem to be covered by update_vsyscall() already.
I tried hard and can't reproduce this behaviour.

Now that I am thinking, maybe we could come up something similar to vsyscall?

If we start by making vsyscall data point to the same object, we may get it
for free. Or am I missing something ?