On Tue, May 10, 2011 at 12:18:02PM -0700, john stultz wrote:
> On Mon, 2011-05-09 at 13:40 -0700, Greg KH wrote:
> > On Fri, May 06, 2011 at 09:08:34AM +0200, Andi Kleen wrote:
> > > > @@ -168,8 +168,13 @@ int __read_mostly timekeeping_suspended;
> > > > static struct timespec xtime_cache __attribute__ ((aligned (16)));
> > > > void update_xtime_cache(u64 nsec)
> > > > {
> > > > - xtime_cache = xtime;
> > > > - timespec_add_ns(&xtime_cache, nsec);
> > > > + /*
> > > > + * Use temporary variable so get_seconds() cannot catch
> > > > + * an intermediate xtime_cache.tv_sec value.
> > > > + */
> > > > + struct timespec ts = xtime;
> > > > + timespec_add_ns(&ts, nsec);
> > >
> > > There's nothing to stop the compiler's optimizer to use memory accesses
> > > directly again. Something similar to ACCESS_ONCE (that is essentially a
> > > volatile cast) may be required to be fully future proof.
> >
> > I agree, this doesn't look like a viable solution at all.
>
> So does something like this look better? I'm really not that familiar
> with how ACCESS_ONCE is to be used, so forgive me if I'm not using it
> correctly.
>
> Signed-off-by: John Stultz <[email protected]>
>
> Index: linux-2.6.32.y/kernel/time/timekeeping.c
> ===================================================================
> --- linux-2.6.32.y.orig/kernel/time/timekeeping.c 2011-05-04
> 19:34:21.604314152 -0700
> +++ linux-2.6.32.y/kernel/time/timekeeping.c 2011-05-10 12:15:48.756236916
> -0700
> @@ -168,8 +168,15 @@ int __read_mostly timekeeping_suspended;
> static struct timespec xtime_cache __attribute__ ((aligned (16)));
> void update_xtime_cache(u64 nsec)
> {
> - xtime_cache = xtime;
> - timespec_add_ns(&xtime_cache, nsec);
> + /*
> + * Use temporary variable so get_seconds() cannot catch
> + * an intermediate xtime_cache.tv_sec value.
> + * The ACCESS_ONCE() keeps the compiler from optimizing
> + * out the intermediate value.
> + */
> + struct timespec ts = xtime;
> + timespec_add_ns(&ts, nsec);
> + ACCESS_ONCE(xtime_cache) = ts;
> }
I think this works, Andi?
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