linux-kernel - RE: [RFC] WRITE_ONCE

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Date:   Sun, 19 Apr 2020 21:37:10 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Petko Manolov' <petko.manolov@...sulko.com>
CC:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: [RFC] WRITE_ONCE_INC() and friends

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.

> 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.

> I merely proposed the _INC() variant for better readability.

More like shorter code lines :-)

	David

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