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Message-ID: <20251218090313.33923750@pumpkin>
Date: Thu, 18 Dec 2025 09:03:13 +0000
From: David Laight <david.laight.linux@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Joel Fernandes
<joel@...lfernandes.org>, "Paul E. McKenney" <paulmck@...nel.org>,
linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>, Michael
Ellerman <mpe@...erman.id.au>, Greg Kroah-Hartman
<gregkh@...uxfoundation.org>, Sebastian Andrzej Siewior
<bigeasy@...utronix.de>, Will Deacon <will@...nel.org>, Peter Zijlstra
<peterz@...radead.org>, Alan Stern <stern@...land.harvard.edu>, John Stultz
<jstultz@...gle.com>, Neeraj Upadhyay <Neeraj.Upadhyay@....com>, Linus
Torvalds <torvalds@...ux-foundation.org>, Andrew Morton
<akpm@...ux-foundation.org>, Frederic Weisbecker <frederic@...nel.org>,
Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
Steven Rostedt <rostedt@...dmis.org>, Lai Jiangshan
<jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>, Ingo Molnar
<mingo@...hat.com>, Waiman Long <longman@...hat.com>, Mark Rutland
<mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>, Vlastimil
Babka <vbabka@...e.cz>, maged.michael@...il.com, Mateusz Guzik
<mjguzik@...il.com>, Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev, Gary Guo
<gary@...yguo.net>, Nikita Popov <github@...pov.com>, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH v4 1/4] compiler.h: Introduce ptr_eq() to preserve
address dependency
On Wed, 17 Dec 2025 20:45:28 -0500
Mathieu Desnoyers <mathieu.desnoyers@...icios.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@...ux-foundation.org>
> Suggested-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Tested-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Acked-by: "Paul E. McKenney" <paulmck@...nel.org>
> Acked-by: Alan Stern <stern@...land.harvard.edu>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: John Stultz <jstultz@...gle.com>
> Cc: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Uladzislau Rezki <urezki@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Lai Jiangshan <jiangshanlai@...il.com>
> Cc: Zqiang <qiang.zhang1211@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: maged.michael@...il.com
> Cc: Mateusz Guzik <mjguzik@...il.com>
> Cc: Gary Guo <gary@...yguo.net>
> Cc: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
> Cc: rcu@...r.kernel.org
> Cc: linux-mm@...ck.org
> Cc: lkmm@...ts.linux.dev
> Cc: Nikita Popov <github@...pov.com>
> Cc: llvm@...ts.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__)
>
> /**
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