Re: [RFC] WRITE_ONCE_INC() and friends

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

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?

							Thanx, Paul