Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency

Alan Stern
2024-09-30T16:43:59Z

On Mon, Sep 30, 2024 at 01:26:53PM +0200, Jonas Oberhauser wrote:
>
>
> Am 9/28/2024 um 4:49 PM schrieb Alan Stern:
> > On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote:
> > > Compiler CSE and SSA GVN optimizations can cause the address dependency
> > > of addresses returned by rcu_dereference to be lost when comparing those
> > > pointers with either constants or previously loaded pointers.
> > >
> > > Introduce ptr_eq() to compare two addresses while preserving the address
> > > dependencies for later use of the address. It should be used when
> > > comparing an address returned by rcu_dereference().
> > >
> > > This is needed to prevent the compiler CSE and SSA GVN optimizations
> > > from replacing the registers holding @a or @b based on their
> >
> > "Replacing" isn't the right word. What the compiler does is use one
> > rather than the other. Furthermore, the compiler can play these games
> > even with values that aren't in registers.
> >
> > You should just say: "... from using @a (or @b) in places where the
> > source refers to @b (or @a) (based on the fact that after the
> > comparison, the two are known to be equal), which does not ..."
>
> I should also point out that it is not enough to prevent the compiler from
> using @a instead of @b.
>
> It must also be prevented from assigning @b=@a, which it is often allowed to
> do after finding @a==@b.

Wouldn't that be a bug? Consider this litmus test:

int x = 0;
int y = 45;
int z = 0;

void P0(int *x, int *y, int *z) {
int r1, r2;

r1 = READ_ONCE(*x);
r2 = READ_ONCE(*y);
if (r1 == r2) {
WRITE_ONCE(*z, 1);
// L1: WRITE_ONCE(*y, r1);
}
}

void P1(int *x, int *y) {
int r3;

WRITE_ONCE(*x, 45);
WRITE_ONCE(*y, 56);
r3 = READ_ONCE(*y);
}

exists (z=1 /\ 1:r3=45) (* Not allowed *)

If the compiler were to make the extra assignment (basically,
uncommenting the line marked L1) then the exists clause could be
satisfied. That would indicate there's a bug in the compiler.

Alan