Re: [RFC PATCH v4 1/4] compiler.h: Introduce ptr_eq() to preserve address dependency

[Mathieu Desnoyers]

On 2025-12-18 04:03, David Laight wrote:
[...]
> > + *
> > + * The compiler barrier() is ineffective at fixing this issue. It does
> > + * not prevent the compiler CSE from losing the address dependency:
> > + *
> > + * int fct_2_volatile_barriers(void)
> > + * {
> > + *     int *a, *b;
> > + *
> > + *     do {
> > + *         a = READ_ONCE(p);
> > + *         asm volatile ("" : : : "memory");
> > + *         b = READ_ONCE(p);
> > + *     } while (a != b);
> > + *     asm volatile ("" : : : "memory");  <-- barrier()
> > + *     return *b;
> > + * }
> > + *
> > + * With gcc 14.2 (arm64):
> > + *
> > + * fct_2_volatile_barriers:
> > + *         adrp    x0, .LANCHOR0
> > + *         add     x0, x0, :lo12:.LANCHOR0
> > + * .L2:
> > + *         ldr     x1, [x0]  <-- x1 populated by first load.
> > + *         ldr     x2, [x0]
> > + *         cmp     x1, x2
> > + *         bne     .L2
> > + *         ldr     w0, [x1]  <-- x1 is used for access which should depend on b.
> > + *         ret
> > + *
> > + * On weakly-ordered architectures, this lets CPU speculation use the
> > + * result from the first load to speculate "ldr w0, [x1]" before
> > + * "ldr x2, [x0]".
> > + * Based on the RCU documentation, the control dependency does not
> > + * prevent the CPU from speculating loads.
>
> I'm not sure that example (of something that doesn't work) is really necessary.
> The simple example of, given:
> 	return a == b ? *a : 0;
> the generated code might speculatively dereference 'b' (not a) before returning
> zero when the pointers are different.

In the past discussion that led to this new API, AFAIU, Linus made it
clear that this counter example needs to be in a comment:

https://lore.kernel.org/lkml/CAHk-=wgBgh5U+dyNaN=+XCdcm2OmgSRbcH4Vbtk8i5ZDGwStSA@xxxxxxxxxxxxxx/

This counter-example is what convinced him that this addresses a real
issue.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com