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

[john stultz]

On Fri, Oct 1, 2010 at 6:09 AM, Glauber Costa <glommer@xxxxxxxxxx> 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?

thanks
-john
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