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

[john stultz]

On Thu, 2011-05-05 at 07:44 +0200, Eric Dumazet wrote:
> Le mercredi 04 mai 2011 Ã 19:54 -0700, john stultz a Ãcrit :
> > On Tue, 2011-05-03 at 20:52 -0700, Andi Kleen wrote:
> > > John Stultz <john.stultz@xxxxxxxxxx> writes:
> > > 
> > > > From: John Stultz <johnstul@xxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxx>
> > 
> > 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.

Well, 32bit/64bit, the time_t .tv_sec portion is a long, so it should be
written atomically. 

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

Fair enough. Such a comment is an improvement.

> > +	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.

Would a comment to the effect of "ensure processors don't re-order calls
to get_seconds" help, or is it still too opaque (or even still
nonsense?).


thanks
-john


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