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

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

[Mathieu Desnoyers]

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@linux-foundation.org>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: John Stultz <jstultz@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Zqiang <qiang.zhang1211@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: maged.michael@gmail.com
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: rcu@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: lkmm@lists.linux.dev
Cc: Nikita Popov <github@npopov.com>
Cc: llvm@lists.linux.dev
Cc: Lian Wang <lianux.mm@gmail.com>
Cc: Kunwu Chan <kunwu.chan@gmail.com>
---
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 +++++++++
 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)
+{
+	OPTIMIZER_HIDE_VAR(a);
+	OPTIMIZER_HIDE_VAR(b);
+	return a == b;
+}
+
+#endif	/* __ASM_GENERIC_PTREQ_H */
diff --git a/include/linux/ptreq.h b/include/linux/ptreq.h
new file mode 100644
index 000000000000..5499f65da30a
--- /dev/null
+++ b/include/linux/ptreq.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_PTREQ_H
+#define _LINUX_PTREQ_H
+
+/*
+ * 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.
+ *
+ * Return value: true if pointers are equal, false otherwise.
+ *
+ * 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.
+ */
+
+#include <asm/ptreq.h>
+
+#endif	/* _LINUX_PTREQ_H */
-- 
2.43.0

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

[Boqun Feng]

On Fri, Oct 09, 2026 at 02:21:33PM -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 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@linux-foundation.org>
> Suggested-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: John Stultz <jstultz@google.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Zqiang <qiang.zhang1211@gmail.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: maged.michael@gmail.com
> Cc: Mateusz Guzik <mjguzik@gmail.com>
> Cc: Gary Guo <gary@garyguo.net>
> Cc: rcu@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: lkmm@lists.linux.dev
> Cc: Nikita Popov <github@npopov.com>
> Cc: llvm@lists.linux.dev
> Cc: Lian Wang <lianux.mm@gmail.com>
> Cc: Kunwu Chan <kunwu.chan@gmail.com>
> ---
> Changes since v1:
> - Move to linux/ptreq.h, implement x86-specific comparison

I prefer the name ptr_eq.h over ptreq.h :)

With that name fix or there is a strong reason of ptreq.h:

Reviewed-by: Boqun Feng <boqun@kernel.org>

Regards,
Boqun

>   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 +++++++++
>  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)
> +{
> +	OPTIMIZER_HIDE_VAR(a);
> +	OPTIMIZER_HIDE_VAR(b);
> +	return a == b;
> +}
> +
> +#endif	/* __ASM_GENERIC_PTREQ_H */
> diff --git a/include/linux/ptreq.h b/include/linux/ptreq.h
> new file mode 100644
> index 000000000000..5499f65da30a
> --- /dev/null
> +++ b/include/linux/ptreq.h
> @@ -0,0 +1,63 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_PTREQ_H
> +#define _LINUX_PTREQ_H
> +
> +/*
> + * 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.
> + *
> + * Return value: true if pointers are equal, false otherwise.
> + *
> + * 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.
> + */
> +
> +#include <asm/ptreq.h>
> +
> +#endif	/* _LINUX_PTREQ_H */
> -- 
> 2.43.0
> 

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

[Mathieu Desnoyers]

On 2026-10-09 15:03, Boqun Feng wrote:
> On Fri, Oct 09, 2026 at 02:21:33PM -0400, Mathieu Desnoyers wrote:
[...]
>>
>> 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.
>>
[...]
> 
> I prefer the name ptr_eq.h over ptreq.h :)
> 
> With that name fix or there is a strong reason of ptreq.h:
> 
> Reviewed-by: Boqun Feng <boqun@kernel.org>
I took rwonce.h (implementing READ_ONCE()/WRITE_ONCE()) as an
inspiration for the file name. I'm open to change it to whatever
people think best. I have no strong preference for either ptreq.h
or ptr_eq.h. I'll let people chime in before respinning an updated
series if a file name change is needed.

Thanks!

Mathieu

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

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@linux-foundation.org>
> Suggested-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: John Stultz <jstultz@google.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Zqiang <qiang.zhang1211@gmail.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: maged.michael@gmail.com
> Cc: Mateusz Guzik <mjguzik@gmail.com>
> Cc: Gary Guo <gary@garyguo.net>
> Cc: rcu@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: lkmm@lists.linux.dev
> Cc: Nikita Popov <github@npopov.com>
> Cc: llvm@lists.linux.dev
> Cc: Lian Wang <lianux.mm@gmail.com>
> Cc: Kunwu Chan <kunwu.chan@gmail.com>
> ---
> 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

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

[Mathieu Desnoyers]

On 2026-10-09 17:55, Gary Guo wrote:
> 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@linux-foundation.org>
>> Suggested-by: Boqun Feng <boqun.feng@gmail.com>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> Cc: "Paul E. McKenney" <paulmck@kernel.org>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Boqun Feng <boqun.feng@gmail.com>
>> Cc: Alan Stern <stern@rowland.harvard.edu>
>> Cc: John Stultz <jstultz@google.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Boqun Feng <boqun.feng@gmail.com>
>> Cc: Frederic Weisbecker <frederic@kernel.org>
>> Cc: Joel Fernandes <joel@joelfernandes.org>
>> Cc: Josh Triplett <josh@joshtriplett.org>
>> Cc: Uladzislau Rezki <urezki@gmail.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
>> Cc: Zqiang <qiang.zhang1211@gmail.com>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Waiman Long <longman@redhat.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: maged.michael@gmail.com
>> Cc: Mateusz Guzik <mjguzik@gmail.com>
>> Cc: Gary Guo <gary@garyguo.net>
>> Cc: rcu@vger.kernel.org
>> Cc: linux-mm@kvack.org
>> Cc: lkmm@lists.linux.dev
>> Cc: Nikita Popov <github@npopov.com>
>> Cc: llvm@lists.linux.dev
>> Cc: Lian Wang <lianux.mm@gmail.com>
>> Cc: Kunwu Chan <kunwu.chan@gmail.com>
>> ---
>> 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?

If architectures end up needing the likely/unlikely variants we can
do that if it saves instructions on the hot path. How about we do this
change when we introduce optimized support for riscv ?

Thanks,

Mathieu


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

[PATCH hazptr v2 3/4] Documentation: RCU: Refer to ptr_eq()

[Mathieu Desnoyers]

Refer to ptr_eq() in the rcu_dereference() documentation.

ptr_eq() is a mechanism that preserves address dependencies when
comparing pointers, and should be favored when comparing a pointer
obtained from rcu_dereference() against another pointer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: John Stultz <jstultz@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Zqiang <qiang.zhang1211@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: maged.michael@gmail.com
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: rcu@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: lkmm@lists.linux.dev
Cc: Nikita Popov <github@npopov.com>
Cc: llvm@lists.linux.dev
Cc: Lian Wang <lianux.mm@gmail.com>
Cc: Kunwu Chan <kunwu.chan@gmail.com>
---
Changes since v1:
- Include feedback from Paul E. McKenney.

Changes since v0:
- Include feedback from Alan Stern.
---
 Documentation/RCU/rcu_dereference.rst | 38 +++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst
index 5bc3785ebfc2..85b5f79ebf14 100644
--- a/Documentation/RCU/rcu_dereference.rst
+++ b/Documentation/RCU/rcu_dereference.rst
@@ -104,11 +104,12 @@ readers working properly:
 	after such branches, but can speculate loads, which can again
 	result in misordering bugs.
 
--	Be very careful about comparing pointers obtained from
-	rcu_dereference() against non-NULL values.  As Linus Torvalds
-	explained, if the two pointers are equal, the compiler could
-	substitute the pointer you are comparing against for the pointer
-	obtained from rcu_dereference().  For example::
+-	Use operations that preserve address dependencies (such as
+	"ptr_eq()") to compare pointers obtained from rcu_dereference()
+	against non-NULL pointers. As Linus Torvalds explained, if the
+	two pointers are equal, the compiler could substitute the
+	pointer you are comparing against for the pointer obtained from
+	rcu_dereference().  For example::
 
 		p = rcu_dereference(gp);
 		if (p == &default_struct)
@@ -125,6 +126,29 @@ readers working properly:
 	On ARM and Power hardware, the load from "default_struct.a"
 	can now be speculated, such that it might happen before the
 	rcu_dereference().  This could result in bugs due to misordering.
+	Performing the comparison with "ptr_eq()" ensures the compiler
+	does not perform such transformation.
+
+	If the comparison is against another pointer, the compiler is
+	allowed to use either pointer for the following accesses, which
+	loses the address dependency and allows weakly-ordered
+	architectures such as ARM and PowerPC to speculate the
+	address-dependent load before rcu_dereference().  For example::
+
+		p1 = READ_ONCE(gp);
+		p2 = rcu_dereference(gp);
+		if (p1 == p2)  /* BUGGY!!! */
+			do_default(p2->a);
+
+	The compiler can use p1->a rather than p2->a, destroying the
+	address dependency.  Performing the comparison with "ptr_eq()"
+	ensures the compiler preserves the address dependencies.
+	Corrected code::
+
+		p1 = READ_ONCE(gp);
+		p2 = rcu_dereference(gp);
+		if (ptr_eq(p1, p2))
+			do_default(p2->a);
 
 	However, comparisons are OK in the following cases:
 
@@ -204,6 +228,10 @@ readers working properly:
 		comparison will provide exactly the information that the
 		compiler needs to deduce the value of the pointer.
 
+	When in doubt, use operations that preserve address dependencies
+	(such as "ptr_eq()") to compare pointers obtained from
+	rcu_dereference() against non-NULL pointers.
+
 -	Disable any value-speculation optimizations that your compiler
 	might provide, especially if you are making use of feedback-based
 	optimizations that take data collected from prior runs.  Such
-- 
2.43.0