linux-kernel - Re: [RFC] WRITE_ONCE

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Date:   Mon, 20 Apr 2020 19:32:15 +0300
From:   Petko Manolov <petko.manolov@...sulko.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     David Laight <David.Laight@...LAB.COM>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] WRITE_ONCE_INC() and friends

On 20-04-20 08:05:45, 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.

There's pros and cons, as usual.  Yet another macro(s) versus sorter/more 
readable code.  This is why i decided to spam the list (and Paul) - in search 
for another opinion.

> > > 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.

Which would be unfortunate.

I just wish there was C type declaration that would force the compiler to do 
what READ/WRITE_ONCE() does now, but i also know this is too naive... :)


		Petko


> > > I merely proposed the _INC() variant for better readability.
> > 
> > More like shorter code lines :-)
> 
> That too!  ;-)
> 
> 							Thanx, Paul