On Fri, Jun 04, 2021 at 12:09:26PM -0700, Linus Torvalds wrote:

Side note: it is worth noting that my version of "volatile_if()" has
an added little quirk: it _ONLY_ orders the stuff inside the
if-statement.

I do think it's worth not adding new special cases (especially that
"asm goto" hack that will generate worse code than the compiler could
do), but it means that

    x = READ_ONCE(ptr);
    volatile_if (x > 0)
        WRITE_ONCE(*z, 42);

has an ordering, but if you write it as

    x = READ_ONCE(ptr);
    volatile_if (x <= 0)
        return;
    WRITE_ONCE(*z, 42);

then I could in theory see teh compiler doing that WRITE_ONCE() as
some kind of non-control dependency.

This may be a minor point, but can that loophole be closed as follows?

define volatile_if(x) \
	if ((({ _Bool __x = (x); BUILD_BUG_ON(__builtin_constant_p(__x)); __x; }) && \
		({ barrier(); 1; })) || ({ barrier(); 0; }))

(It's now a little later at night than when I usually think about this 
sort of thing, so my brain isn't firing on all its cylinders.  Forgive 
me if this is a dumb question.)

Alan