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

[Paul E. McKenney]

On Wed, May 04, 2011 at 11:21:35PM -0700, john stultz wrote:
> On Thu, 2011-05-05 at 07:44 +0200, Eric Dumazet wrote:
> > 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@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?).

A CPU that reordered syscalls reading from or writing to a given memory
location is broken.  At least if the CPU does such reordering in a way
that lets the software detect it.  There is quite a bit of code out there
that assumes cache coherence, so I sure hope that CPUs don't require
the above memory barrier...

							Thanx, Paul
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