Re: [PATCH -rcu 0/2] kcsan: Improvements to reporting

[Paul E. McKenney]

On Thu, Jan 09, 2020 at 06:03:39PM +0100, Marco Elver wrote:
> On Thu, 9 Jan 2020 at 17:27, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Thu, Jan 09, 2020 at 04:23:20PM +0100, Marco Elver wrote:
> > > Improvements to KCSAN data race reporting:
> > > 1. Show if access is marked (*_ONCE, atomic, etc.).
> > > 2. Rate limit reporting to avoid spamming console.
> > >
> > > Marco Elver (2):
> > >   kcsan: Show full access type in report
> > >   kcsan: Rate-limit reporting per data races
> >
> > Queued and pushed, thank you!  I edited the commit logs a bit, so could
> > you please check to make sure that I didn't mess anything up?
> 
> Looks good to me, thank you.
> 
> > At some point, boot-time-allocated per-CPU arrays might be needed to
> > avoid contention on large systems, but one step at a time.  ;-)
> 
> I certainly hope the rate of fixing/avoiding data races will not be
> eclipsed by the rate at which new ones are introduced. :-)

Me too!

However, on a large system, duplicate reports might happen quite
frequently, which might cause slowdowns given the single global
array.  Or maybe not -- I guess we will find out soon enough. ;-)

But I must confess that I am missing how concurrent access to the
report_times[] array is handled.  I would have expected that
rate_limit_report() would choose a random starting entry and
search circularly.  And I would expect that the code at the end
of that function would instead look something like this:

	if (ktime_before(oldtime, invalid_before) &&
	    cmpxchg(&use_entry->time, oldtime, now) == oldtime) {
		use_entry->frame1 = frame1;
		use_entry->frame2 = frame2;
	} else {
		// Too bad, next duplicate report won't be suppressed.
	}

Where "oldtime" is captured from the entry during the scan, and from the
first entry scanned.  This cmpxchg() approach is of course vulnerable
to the ->frame1 and ->frame2 assignments taking more than three seconds
(by default), but if that becomes a problem, a WARN_ON() could be added:

	if (ktime_before(oldtime, invalid_before) &&
	    cmpxchg(&use_entry->time, oldtime, now) == oldtime) {
		use_entry->frame1 = frame1;
		use_entry->frame2 = frame2;
		WARN_ON_ONCE(use_entry->time != now);
	} else {
		// Too bad, next duplicate report won't be suppressed.
	}

So what am I missing here?

							Thanx, Paul

> Thanks,
> -- Marco
> 
> >                                                         Thanx, Paul
> >
> > >  kernel/kcsan/core.c   |  15 +++--
> > >  kernel/kcsan/kcsan.h  |   2 +-
> > >  kernel/kcsan/report.c | 153 +++++++++++++++++++++++++++++++++++-------
> > >  lib/Kconfig.kcsan     |  10 +++
> > >  4 files changed, 148 insertions(+), 32 deletions(-)
> > >
> > > --
> > > 2.25.0.rc1.283.g88dfdc4193-goog
> > >
> >
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