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(mtl.efficios.com [216.120.195.104]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4ht87n48FjzhGy; Sun, 27 Sep 2026 11:51:49 -0400 (EDT) From: Mathieu Desnoyers To: "Paul E . McKenney" Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Linus Torvalds , Boqun Feng , Joel Fernandes , Alan Stern , Greg Kroah-Hartman , Sebastian Andrzej Siewior , Will Deacon , Peter Zijlstra , John Stultz , Neeraj Upadhyay , Frederic Weisbecker , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Gary Guo , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev, Nikita Popov , llvm@lists.linux.dev Subject: [PATCH hazptr 2/4] compiler.h: Introduce ptr_eq() to preserve address dependency Date: Sun, 27 Sep 2026 11:51:29 -0400 Message-ID: <20260927155134.4740-3-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260927155134.4740-1-mathieu.desnoyers@efficios.com> References: <20260927155134.4740-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 8419B160009 X-Stat-Signature: pf4tzff9wwzbfz9hfw9534m9jz8o3szh X-Rspam-User: X-HE-Tag: 1790524310-293855 X-HE-Meta: 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 XKH9oxvZ 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 gA0ku0XC k0mOKR165n9KwEHfnIRbDeXc0gZOFQ3kf/k89sDGSGw6L7IMIHLQcC4TEC2FtCfsP9J2myB2iXEU+U57VQsFScuKmFphWNrOAQEwhjXewAoZTvQTgdE3+j3N9lpLUN7jqjqyA5A1mkzicstGOdtZ1Hh0oZdqOOjHMP8XbnqbmxCDSyfZl3Vsrw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 Suggested-by: Boqun Feng Signed-off-by: Mathieu Desnoyers Reviewed-by: Boqun Feng Reviewed-by: Joel Fernandes (Google) Tested-by: Joel Fernandes (Google) Acked-by: "Paul E. McKenney" Acked-by: Alan Stern Cc: Greg Kroah-Hartman Cc: Sebastian Andrzej Siewior Cc: "Paul E. McKenney" Cc: Will Deacon Cc: Peter Zijlstra Cc: Boqun Feng Cc: Alan Stern Cc: John Stultz Cc: Neeraj Upadhyay Cc: Linus Torvalds Cc: Boqun Feng Cc: Frederic Weisbecker Cc: Joel Fernandes Cc: Josh Triplett Cc: Uladzislau Rezki Cc: Steven Rostedt Cc: Lai Jiangshan Cc: Zqiang Cc: Ingo Molnar Cc: Waiman Long Cc: Mark Rutland Cc: Thomas Gleixner Cc: Vlastimil Babka Cc: maged.michael@gmail.com Cc: Mateusz Guzik Cc: Gary Guo Cc: Jonas Oberhauser Cc: rcu@vger.kernel.org Cc: linux-mm@kvack.org Cc: lkmm@lists.linux.dev Cc: Nikita Popov 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 70cb31d60538..0a8c8e53cdad 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -166,6 +166,69 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, __PASTE(name, \ __PASTE(_, __COUNTER__))) +/* + * 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. + */ +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; +} + /** * data_race - mark an expression as containing intentional data races * -- 2.43.0