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SubjectRe: [PATCH] kcsan: Add option to allow watcher interruptions
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?

> 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.

Thanx, Paul

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