Re: [PATCH] time: Add locking to xtime access in get_seconds()

Eric Dumazet
2011-05-05T05:44:12Z

Le mercredi 04 mai 2011 à 19:54 -0700, john stultz a écrit :
> On Tue, 2011-05-03 at 20:52 -0700, Andi Kleen wrote:
> > John Stultz <john.stultz@linaro.org> writes:
> >
> > > From: John Stultz <johnstul@us.ibm.com>
> > >
> > > So get_seconds() has always been lock free, with the assumption
> > > that accessing a long will be atomic.
> > >
> > > However, recently I came across an odd bug where time() access could
> > > occasionally be inconsistent, but only on power7 hardware. The
> >
> > Shouldn't a single rmb() be enough to avoid that?
> >
> > If not then I suspect there's a lot more code buggy on that CPU than
> > just the time.
>
> So interestingly, I've found that the issue was not as complex as I
> first assumed. While the rmb() is probably a good idea for
> get_seconds(), but it alone does not solve the issue I was seeing,
> making it clear my theory wasn't correct.
>
> The problem was reported against the 2.6.32-stable kernel, and had not
> been seen in later kernels. I had assumed the change to logarithmic time
> accumulation basically reduced the window for for the issue to be seen,
> but it would likely still show up eventually.
>
> When the rmb() alone did not solve this issue, I looked to see why the
> locking did resolve it, and then it was clear: The old
> update_xtime_cache() function doesn't set the xtime_cache values
> atomically.
>
> Now, the xtime_cache writing is done under the xtime_lock, so the
> get_seconds() locking resolves the issue, but isn't appropriate since
> get_seconds() is called from machine check handlers.
>
> So the fix here for the 2.6.32-stable tree is to just update xtime_cache
> in one go as done with the following patch.
>
> I also added the rmb() for good measure, and the rmb() should probably
> also go upstream since theoretically there maybe a platform that could
> do out of order syscalls.
>
> I suspect the reason this hasn't been triggered on x86 or power6 is due
> to compiler or processor optimizations reordering the assignment to in
> effect make it atomic. Or maybe the timing window to see the issue is
> harder to observe?
>
>
> Signed-off-by: John Stultz <johnstul@us.ibm.com>
>
> 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 2011-05-04 19:39:09.972203989 -0700
> @@ -168,8 +168,10 @@ 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 timespec so xtime_cache is updated atomically */

Atomically is not possible on 32bit platform, so this comment is
misleading.

What about a comment saying :
/*
* use temporary variable so get_seconds() cannot catch
* intermediate value (one second backward)
*/

> + struct timespec ts = xtime;
> + timespec_add_ns(&ts, nsec);
> + xtime_cache = ts;
> }
>
> /* must hold xtime_lock */
> @@ -859,6 +861,7 @@ EXPORT_SYMBOL_GPL(monotonic_to_bootbased
>
> unsigned long get_seconds(void)
> {
> + rmb();

Please dont, this makes no sense, and with no comment anyway.

> return xtime_cache.tv_sec;
> }
> EXPORT_SYMBOL(get_seconds);
>