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

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Date:	Wed, 13 Oct 2010 11:07:46 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, avi@...hat.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] use a stable clock reference in vdso vgetns

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@...hat.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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