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(mtl.efficios.com [216.120.195.104]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4ht87n5L08zhH0; Sun, 27 Sep 2026 11:51:49 -0400 (EDT) From: Mathieu Desnoyers To: "Paul E . McKenney" Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Alan Stern , Joel Fernandes , Greg Kroah-Hartman , Sebastian Andrzej Siewior , Will Deacon , Peter Zijlstra , Boqun Feng , John Stultz , Neeraj Upadhyay , Linus Torvalds , 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 3/4] Documentation: RCU: Refer to ptr_eq() Date: Sun, 27 Sep 2026 11:51:30 -0400 Message-ID: <20260927155134.4740-4-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: rspam05 X-Rspamd-Queue-Id: A2BD01C0002 X-Stat-Signature: 95zyy5ef6gzhppg51ci69stfjrumstmn X-Rspam-User: X-HE-Tag: 1790524310-205023 X-HE-Meta: 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 jGc2/Cwo 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 E9TyzfIS EDO7EKYmIf8wAOztvuY65b2YSW03wBRcx88uqYDdv6+woqkVU7uHwwaMqB9hQg8U6Apde3ZAtvsweIyRIlBvbhlCpVbeWUhn70xCRemTXSiPBWeZkB8R9iR1BspkM/suyFDfweLVAqqVQYZ3X5LFVQ25KGZDOmj4HgF5xU7LJ7kcCJRq//LvPQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 Acked-by: Alan Stern Acked-by: Paul E. McKenney Reviewed-by: Joel Fernandes (Google) 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 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