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

[David Laight <david.laight.linux@gmail.com>]

On Wed, 17 Dec 2025 20:45:28 -0500
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> 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.
> 
> 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>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Acked-by: "Paul E. McKenney" <paulmck@kernel.org>
> Acked-by: Alan Stern <stern@rowland.harvard.edu>
> 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: Neeraj Upadhyay <Neeraj.Upadhyay@amd.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: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
> 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
> ---
> Changes since v0:
> - Include feedback from Alan Stern.
> ---
>  include/linux/compiler.h | 63 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 63 insertions(+)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 5b45ea7dff3e..c5ca3b54c112 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -163,6 +163,69 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  	__asm__ ("" : "=r" (var) : "0" (var))
>  #endif
>  
> +/*
> + * 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.

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.

	David

> + */
> +static __always_inline
> +int ptr_eq(const volatile void *a, const volatile void *b)
> +{
> +	OPTIMIZER_HIDE_VAR(a);
> +	OPTIMIZER_HIDE_VAR(b);
> +	return a == b;
> +}
> +
>  #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
>  
>  /**