Re: [RFC] LKMM: Add volatile_if()

[Segher Boessenkool]

On Sun, Jun 06, 2021 at 11:04:49AM -0700, Linus Torvalds wrote:
>     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.

You didn't use a barrier with these semantics though.  There is nothing
in that code that guarantees a branch.

> See, but it VIOLATES the semantics of the code.

The code violates your expectations 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.

You truly should have written a branch in tthe asm if you truly wanted
a branch instruction.


Segher