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Date:   Sun, 26 Jul 2020 18:48:05 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     peterz@infradead.org
Cc:     Marco Elver <elver@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>, jakub@redhat.com,
        hjl.tools@gmail.com
Subject: Re: [PATCH] kcsan: Add option to allow watcher interruptions
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On Sun, Jul 26, 2020 at 01:52:42PM +0200, peterz@infradead.org wrote:
> On Sat, Jul 25, 2020 at 03:07:50PM -0700, Paul E. McKenney wrote:
> > On Sat, Jul 25, 2020 at 10:21:31PM +0200, peterz@infradead.org wrote:
> > > On Sat, Jul 25, 2020 at 10:10:13PM +0200, peterz@infradead.org wrote:
> > > > On Sat, Jul 25, 2020 at 12:39:09PM -0700, Paul E. McKenney wrote:
> > > 
> > > > > This gets me the following for __rcu_read_lock():
> > > > > 
> > > > > 00000000000000e0 <__rcu_read_lock>:
> > > > >       e0:	48 8b 14 25 00 00 00 	mov    0x0,%rdx
> > > > >       e7:	00 
> > > > >       e8:	8b 82 e0 02 00 00    	mov    0x2e0(%rdx),%eax
> > > > >       ee:	83 c0 01             	add    $0x1,%eax
> > > > >       f1:	89 82 e0 02 00 00    	mov    %eax,0x2e0(%rdx)
> > > > >       f7:	c3                   	retq   
> > > > >       f8:	0f 1f 84 00 00 00 00 	nopl   0x0(%rax,%rax,1)
> > > > >       ff:	00 
> > > > > 
> > > > > One might hope for a dec instruction, but this isn't bad.  We do lose
> > > > > a few instructions compared to the C-language case due to differences
> > > > > in address calculation:
> > > > > 
> > > > > 00000000000000e0 <__rcu_read_lock>:
> > > > >       e0:	48 8b 04 25 00 00 00 	mov    0x0,%rax
> > > > >       e7:	00 
> > > > >       e8:	83 80 e0 02 00 00 01 	addl   $0x1,0x2e0(%rax)
> > > > >       ef:	c3                   	retq   
> > > > 
> > > > Shees, that's daft... I think this is one of the cases where GCC is
> > > > perhaps overly cautious when presented with 'volatile'.
> > > > 
> > > > It has a history of generating excessively crap code around volatile,
> > > > and while it has improved somewhat, this seems to show there's still
> > > > room for improvement...
> > > > 
> > > > I suppose this is the point where we go bug a friendly compiler person.
> > 
> > Sounds very good!  Do you have someone specific in mind?
> 
> Jakub perhaps?, Cc'ed
> 
> > > Having had a play with godbolt.org, it seems clang isn't affected by
> > > this particular flavour of crazy, but GCC does indeed refuse to fuse the
> > > address calculation and the addition.
> > 
> > So there is hope, then!
> > 
> > And even GCC's current state is an improvement.  Last I messed with this,
> > the ACCESS_ONCE()++ approach generated a load, a register increment,
> > and a store.
> > 
> > Do you still have the godbolt.org URLs?  I would be happy to file
> > a bugzilla.
> 
> https://godbolt.org/z/rP8rYM

Here you go!  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96327

							Thanx, Paul