On Mon, 7 Jun 2021 at 10:02, Alexander Monakov [off-list ref] wrote:

On Sun, 6 Jun 2021, Linus Torvalds wrote:

[...]

quoted

On Sun, Jun 6, 2021 at 2:19 PM Alexander Monakov [off-list ref] wrote:

[...]

quoted

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