[PATCH 2/2] Documentation: RCU: Refer to ptr_eq()

Mathieu Desnoyers
2024-09-28T13:53:53Z

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>
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
---
Documentation/RCU/rcu_dereference.rst | 34 +++++++++++++++++++++++----
1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst
index 2524dcdadde2..c36b8d1721f6 100644
--- a/Documentation/RCU/rcu_dereference.rst
+++ b/Documentation/RCU/rcu_dereference.rst
@@ -104,11 +104,13 @@ 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 relational operators which preserve address dependencies
+ (such as "ptr_eq()") to compare pointers obtained from
+ rcu_dereference() against non-NULL values or against pointers
+ obtained from prior loads. 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 +127,23 @@ 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 a pointer obtained from prior
+ loads, the compiler is allowed to use either register 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)
+ do_default(p2->a);
+
+ Performing the comparison with "ptr_eq()" ensures the compiler
+ preserves the address dependencies.

However, comparisons are OK in the following cases:

@@ -204,6 +223,11 @@ readers working properly:
comparison will provide exactly the information that the
compiler needs to deduce the value of the pointer.

+ When in doubt, use relational operators that preserve address
+ dependencies (such as "ptr_eq()") to compare pointers obtained
+ from rcu_dereference() against non-NULL values or against
+ pointers obtained from prior loads.
+
- 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
--
2.39.2