Re: [PATCH 3/6] bitops: define gen_test_bit() the same way as the rest of functions

[Alexander Lobakin]

From: Marco Elver <elver@xxxxxxxxxx>
Date: Tue, 7 Jun 2022 15:43:49 +0200

> On Mon, Jun 06, 2022 at 01:49PM +0200, Alexander Lobakin wrote:
> > Currently, the generic test_bit() function is defined as a one-liner
> > and in case with constant bitmaps the compiler is unable to optimize
> > it to a constant. At the same time, gen_test_and_*_bit() are being
> > optimized pretty good.
> > Define gen_test_bit() the same way as they are defined.
> > 
> > Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
> > ---
> >  include/asm-generic/bitops/generic-non-atomic.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/asm-generic/bitops/generic-non-atomic.h b/include/asm-generic/bitops/generic-non-atomic.h
> > index 7a60adfa6e7d..202d8a3b40e1 100644
> > --- a/include/asm-generic/bitops/generic-non-atomic.h
> > +++ b/include/asm-generic/bitops/generic-non-atomic.h
> > @@ -118,7 +118,11 @@ gen___test_and_change_bit(unsigned int nr, volatile unsigned long *addr)
> >  static __always_inline int
> >  gen_test_bit(unsigned int nr, const volatile unsigned long *addr)
> >  {
> > -	return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
> > +	const unsigned long *p = (const unsigned long *)addr + BIT_WORD(nr);
> > +	unsigned long mask = BIT_MASK(nr);
> > +	unsigned long val = *p;
> > +
> > +	return !!(val & mask);
> 
> Unfortunately this makes the dereference of 'addr' non-volatile, and
> effectively weakens test_bit() to the point where I'd no longer consider
> it atomic. Per atomic_bitops.txt, test_bit() is atomic.
> 
> The generic version has been using a volatile access to make it atomic
> (akin to generic READ_ONCE() casting to volatile). The volatile is also
> the reason the compiler can't optimize much, because volatile forces a
> real memory access.

Ah-ha, I see now. Thanks for catching and explaining this!

> 
> Yes, confusingly, test_bit() lives in non-atomic.h, and this had caused
> confusion before, but the decision was made that moving it will cause
> headaches for ppc so it was left alone:
> https://lore.kernel.org/all/87a78xgu8o.fsf@xxxxxxxxxxxxxxxxxxxxxxx/T/#u
> 
> As for how to make test_bit() more compiler-optimization friendly, I'm
> guessing that test_bit() needs some special casing where even the
> generic arch_test_bit() is different from the gen_test_bit().
> gen_test_bit() should probably assert that whatever it is called with
> can actually be evaluated at compile-time so it is never accidentally
> used otherwise.

I like the idea! Will do in v2.
I can move the generics and after, right below them, define
'const_*' helpers which will mostly redirect to 'generic_*', but
for test_bit() it will be a separate function with no `volatile`
and with an assertion that the input args are constants.

> 
> I would also propose adding a comment close to the deref that test_bit()
> is atomic and the deref needs to remain volatile, so future people will
> not try to do the same optimization.

I think that's also the reason why it's not underscored, right?

> 
> Thanks,
> -- Marco

Thanks,
Olek