Re: [RFC] LKMM: Add volatile_if()

Alan Stern
2021-06-06T18:23:10Z

On Sun, Jun 06, 2021 at 11:04:49AM -0700, Linus Torvalds wrote:
> On Sun, Jun 6, 2021 at 4:56 AM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> >
> > And that is a simple fact, since the same assembler code (at the same
> > spot in the program) will do the same thing no matter how that ended up
> > there.
>
> The thing is, that's exactl;y what gcc violates.
>
> The example - you may not have been cc'd personally on that one - was
> something like
>
> if (READ_ONCE(a)) {
> barrier();
> WRITE_ONCE(b,1);
> } else {
> barrier();
> WRITE_ONCE(b, 1);
> }
>
> and currently because gcc thinks "same exact code", it will actually
> optimize this to (pseudo-asm):
>
> LD A
> "empty asm"
> ST $1,B
>
> which is very much NOT equivalent to
>
> LD A
> BEQ over
> "empty asm"
> ST $1,B
> JMP join
>
> over:
> "empty asm"
> ST $1,B
>
> join:
>
> and that's the whole point of the barriers.
>
> It's not equivalent exactly because of memory ordering. In the first
> case, there is no ordering on weak architectures. In the second case,
> there is always an ordering, because of CPU consistency guarantees.
>
> And no, gcc doesn't understand about memory ordering. But that's
> exactly why we use inline asms.
>
> > And the compiler always is allowed to duplicate, join, delete, you name
> > it, inline assembler code. The only thing that it cares about is
> > semantics of the code, just like for any other code.
>
> See, but it VIOLATES the semantics of the code.
>
> You can't join those two empty asm's (and then remove the branch),
> because the semantics of the code really aren't the same any more if
> you do. Truly.

To be fair, the same argument applies even without the asm code. The
compiler will translate

if (READ_ONCE(a))
WRITE_ONCE(b, 1);
else
WRITE_ONCE(b, 1);

to

LD A
ST $1,B

intstead of

LD A
BEQ over
ST $1,B
JMP join

over:
ST $1,B

join:

And these two are different for the same memory ordering reasons as
above.

Alan