Re: [RFC] WRITE_ONCE_INC() and friends

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Apr 21, 2020 at 11:33:57AM +0200, Marco Elver wrote:
> On Tue, 21 Apr 2020 at 01:12, Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Tue, Apr 21, 2020 at 12:57:15AM +0200, Marco Elver wrote:
> > > On Mon, 20 Apr 2020, Paul E. McKenney wrote:
> > >
> > > > On Sun, Apr 19, 2020 at 09:37:10PM +0000, David Laight wrote:
> > > > > From: Petko Manolov
> > > > > > Sent: 19 April 2020 19:30
> > > > > >
> > > > > > On 20-04-19 18:02:50, David Laight wrote:
> > > > > > > From: Petko Manolov
> > > > > > > > Sent: 19 April 2020 10:45
> > > > > > > > Recently I started reading up on KCSAN and at some point I ran into stuff like:
> > > > > > > >
> > > > > > > > WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1);
> > > > > > > > WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1);
> > > > > > >
> > > > > > > If all the accesses use READ/WRITE_ONCE() why not just mark the structure
> > > > > > > field 'volatile'?
> > > > > >
> > > > > > This is a bit heavy.  I guess you've read this one:
> > > > > >
> > > > > >         https://lwn.net/Articles/233479/
> > > > >
> > > > > I remember reading something similar before.
> > > > > I also remember a very old gcc (2.95?) that did a readback
> > > > > after every volatile write on sparc (to flush the store buffer).
> > > > > That broke everything.
> > > > >
> > > > > I suspect there is a lot more code that is attempting to be lockless
> > > > > these days.
> > > > > Ring buffers (one writer and one reader) are a typical example where
> > > > > you don't need locks but do need to use a consistent value.
> > > > >
> > > > > Now you may also need ordering between accesses - which I think needs
> > > > > more than volatile.
> > > >
> > > > In Petko's patch, all needed ordering is supplied by the fact that it
> > > > is the same variable being read and written.  But yes, in many other
> > > > cases, more ordering is required.
> > > >
> > > > > > And no, i am not sure all accesses are through READ/WRITE_ONCE().  If, for
> > > > > > example, all others are from withing spin_lock/unlock pairs then we _may_ not
> > > > > > need READ/WRITE_ONCE().
> > > > >
> > > > > The cost of volatile accesses is probably minimal unless the
> > > > > code is written assuming the compiler will only access things once.
> > > >
> > > > And there are variables marked as volatile, for example, jiffies.
> > > >
> > > > But one downside of declaring variables volatile is that it can prevent
> > > > KCSAN from spotting violations of the concurrency design for those
> > > > variables.
> > >
> > > Note that, KCSAN currently treats volatiles not as special, except a
> > > list of some known global volatiles (like jiffies). This means, that
> > > KCSAN will tell us about data races involving unmarked volatiles (unless
> > > they're in the list).
> > >
> > > As far as I can tell, this is what we want. At least according to LKMM.
> > >
> > > If, for whatever reason, volatiles should be treated differently, we'll
> > > have to modify the compilers to emit different instrumentation for the
> > > kernel.
> >
> > I stand corrected, then, thank you!
> >
> > In the current arrangement, declaring a variable volatile will cause
> > KCSAN to generate lots of false positives.
> >
> > I don't currently have a strong feeling on changing the current situation
> > with respect to volatile variables.  Is there a strong reason to change?
> > The general view of the community, as you say, has been that you don't use
> > the volatile keyword outside of exceptions such as jiffies, atomic_read(),
> > atomic_set(), READ_ONCE(), WRITE_ONCE() and perhaps a few others.
> >
> > Thoughts?
> 
> I certainly agree, and also want to point out that checkpatch.pl
> complains about volatile. We know using volatile has problems. KCSAN
> is (along with checkpatch.pl) another tool that can warn us about such
> problems (warning in case there is real concurrency). Another thing to
> point out is that volatile is not portable, in case
> READ_ONCE()/WRITE_ONCE()'s smp_load_barrier_depends() is not a noop.
> So from what I see, there are strong reasons against changing the
> situation for volatiles and KCSAN.

All good points, thank you!

							Thanx, Paul