[RFC PATCH v2 2/4] Documentation: RCU: Refer to ptr_eq()

[Mathieu Desnoyers]

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@xxxxxxxxxxxx>
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: John Stultz <jstultz@xxxxxxxxxx>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Zqiang <qiang.zhang1211@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: maged.michael@xxxxxxxxx
Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Gary Guo <gary@xxxxxxxxxxx>
Cc: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx>
Cc: rcu@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: lkmm@xxxxxxxxxxxxxxx
Cc: Nikita Popov <github@xxxxxxxxxx>
Cc: llvm@xxxxxxxxxxxxxxx
---
Changes since v0:
- Include feedback from Alan Stern.

Changes since v1:
- Include feedback from Paul E. McKenney.
---
 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 2524dcdadde2..de6175bf430f 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
-- 
2.39.2