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

Jonas Oberhauser
2024-09-30T17:05:39Z

Am 9/30/2024 um 6:43 PM schrieb Alan Stern:
> 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?

That's why I said that it is often allowed to do it. In your case it
wouldn't, but it is often possible when a and b are non-atomic &
non-volatile (and haven't escaped, and I believe sometimes even then).

It happens for example here with GCC 14.1.0 -O3:

int fct_hide(void)
{
int *a, *b;

do {
a = READ_ONCE(p);
asm volatile ("" : : : "memory");
b = READ_ONCE(p);
} while (a != b);
OPTIMIZER_HIDE_VAR(b);
return *b;
}

ldr r1, [r2]
ldr r3, [r2]
cmp r1, r3
bne .L6
mov r3, r1 // nay...
ldr r0, [r3] // yay!
bx lr

Have fun,

jonas