Re: [PATCH hazptr v2 2/4] ptreq.h: Introduce ptr_eq() to preserve address dependency

[Gary Guo]

On Fri Oct 9, 2026 at 7:21 PM BST, 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 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 preserve address dependencies and allows the
> following misordering speculations:
>
> - If @b is a constant, the compiler can issue the loads which depend
>   on @a before loading @a.
> - If @b is a register populated by a prior load, weakly-ordered
>   CPUs can speculate loads which depend on @a before loading @a.
>
> The same logic applies with @a and @b swapped.
>
> The header "linux/ptreq.h" is the expected include target. It
> contains the documentation of the ptr_eq() API.
>
> An architecture specific implementation of the pointer comparison
> can be implemented by each architecture as asm/ptreq.h. The x86
> implementation is provided initially. If no implementation header is
> present for the architecture, an arch-agnostic fallback based on
> OPTIMIZER_HIDE_VAR is included from asm-generic.
>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Cc: John Stultz <jstultz@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Zqiang <qiang.zhang1211@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: maged.michael@xxxxxxxxx
> Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
> Cc: Gary Guo <gary@xxxxxxxxxxx>
> Cc: rcu@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: lkmm@xxxxxxxxxxxxxxx
> Cc: Nikita Popov <github@xxxxxxxxxx>
> Cc: llvm@xxxxxxxxxxxxxxx
> Cc: Lian Wang <lianux.mm@xxxxxxxxx>
> Cc: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> ---
> Changes since v1:
> - Move to linux/ptreq.h, implement x86-specific comparison
>   and asm-generic header (fallback).
>
> Changes since v0:
> - Include feedback from Alan Stern.
> ---
>  arch/x86/include/asm/ptreq.h | 16 +++++++++
>  include/asm-generic/Kbuild   |  1 +
>  include/asm-generic/ptreq.h  | 16 +++++++++

I agree with Boqun that this should be ptr_eq. There is no need to save that
character :)

>  include/linux/ptreq.h        | 63 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 96 insertions(+)
>  create mode 100644 arch/x86/include/asm/ptreq.h
>  create mode 100644 include/asm-generic/ptreq.h
>  create mode 100644 include/linux/ptreq.h
>
> diff --git a/arch/x86/include/asm/ptreq.h b/arch/x86/include/asm/ptreq.h
> new file mode 100644
> index 000000000000..593b45f80451
> --- /dev/null
> +++ b/arch/x86/include/asm/ptreq.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_PTREQ_H
> +#define _ASM_X86_PTREQ_H
> +
> +#include <linux/types.h>
> +#include <asm/asm.h>
> +
> +static __always_inline
> +bool ptr_eq(const volatile void *a, const volatile void *b)
> +{
> +	bool ret;
> +	asm(__ASM_SIZE(cmp) " %1,%2" : "=@ccz" (ret) : "r" (a), "r" (b));
> +	return ret;
> +}
> +
> +#endif	/* _ASM_X86_PTREQ_H */
> diff --git a/include/asm-generic/Kbuild b/include/asm-generic/Kbuild
> index 2bc00c67dc54..e1d95dea09d5 100644
> --- a/include/asm-generic/Kbuild
> +++ b/include/asm-generic/Kbuild
> @@ -47,6 +47,7 @@ mandatory-y += percpu.h
>  mandatory-y += percpu_types.h
>  mandatory-y += pgalloc.h
>  mandatory-y += preempt.h
> +mandatory-y += ptreq.h
>  mandatory-y += rqspinlock.h
>  mandatory-y += runtime-const.h
>  mandatory-y += rwonce.h
> diff --git a/include/asm-generic/ptreq.h b/include/asm-generic/ptreq.h
> new file mode 100644
> index 000000000000..db5a88628b1e
> --- /dev/null
> +++ b/include/asm-generic/ptreq.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_GENERIC_PTREQ_H
> +#define __ASM_GENERIC_PTREQ_H
> +
> +#include <linux/types.h>
> +#include <linux/compiler.h>
> +
> +static __always_inline
> +bool ptr_eq(const volatile void *a, const volatile void *b)

I suppose this is `const volatile` to take advantage of implicit conversion that
adds cv-qualifiers to be added implicitly. Might worth a mention in somewhere
(commit message maybe) because I was confused for a second on why this needs
to be volatile.

> +{
> +	OPTIMIZER_HIDE_VAR(a);
> +	OPTIMIZER_HIDE_VAR(b);
> +	return a == b;
> +}

One thought on inline assembly. For RISC-V, branching with comparison is a
single instruction, but there's no instruction for obtaining the boolean for
a == b.

One option is to do

    static __always_inline
    bool ptr_eq_likely(const volatile void *a, const volatile void *b) {
        asm goto ("bne %0, %1, %l2" : : "r" (a), "r" (b) :: not_eq);
        return true;
    not_eq:
        return likely(false);
    }

and then the optimizer can turn a ptr_eq call + a branch to a single
instruction. This, however, would require likely + unlikely variants.

Alternatively, we can just not have all the complexity, and just emit one extra
instruction:

    static __always_inline
    bool ptr_eq(const volatile void *a, const volatile void *b) {
        unsigned long diff;
        asm("xor %0, %1, %2" : "=r" (diff) : "r" (a), "r" (b));
        return diff == 0;
    }

Thoughts?

Best,
Gary