Re: [RFC] LKMM: Add volatile_if()

[Marco Elver <elver@google.com>]

On Mon, 7 Jun 2021 at 10:02, Alexander Monakov <amonakov@ispras.ru> wrote:
> On Sun, 6 Jun 2021, Linus Torvalds wrote:
[...]
> > On Sun, Jun 6, 2021 at 2:19 PM Alexander Monakov <amonakov@ispras.ru> wrote:
[...]
> > Btw, since we have compiler people on line, the suggested 'barrier()'
> > isn't actually perfect for this particular use:
> >
> >    #define barrier() __asm__ __volatile__("" : : "i" (__COUNTER__) : "memory")
> >
> > in the general barrier case, we very much want to have that "memory"
> > clobber, because the whole point of the general barrier case is that
> > we want to make sure that the compiler doesn't cache memory state
> > across it (ie the traditional use was basically what we now use
> > "cpu_relax()" for, and you would use it for busy-looping on some
> > condition).
> >
> > In the case of "volatile_if()", we actually would like to have not a
> > memory clobber, but a "memory read". IOW, it would be a barrier for
> > any writes taking place, but reads can move around it.
> >
> > I don't know of any way to express that to the compiler. We've used
> > hacks for it before (in gcc, BLKmode reads turn into that kind of
> > barrier in practice, so you can do something like make the memory
> > input to the asm be a big array). But that turned out to be fairly
> > unreliable, so now we use memory clobbers even if we just mean "reads
> > random memory".
>
> So the barrier which is a compiler barrier but not a machine barrier is
> __atomic_signal_fence(model), but internally GCC will not treat it smarter
> than an asm-with-memory-clobber today.

FWIW, Clang seems to be cleverer about it, and seems to do the optimal
thing if I use a __atomic_signal_fence(__ATOMIC_RELEASE):
https://godbolt.org/z/4v5xojqaY

Thanks,
-- Marco