Date:   Mon, 24 Jul 2017 14:36:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 5/6] doc: No longer allowed to use rcu_dereference on non-pointers

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/RCU/rcu_dereference.txt | 61 ++++++++++++-----------------------
 1 file changed, 21 insertions(+), 40 deletions(-)

diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt
index b2a613f16d74..1acb26b09b48 100644
--- a/Documentation/RCU/rcu_dereference.txt
+++ b/Documentation/RCU/rcu_dereference.txt
@@ -25,35 +25,35 @@ o	You must use one of the rcu_dereference() family of primitives
 	for an example where the compiler can in fact deduce the exact
 	value of the pointer, and thus cause misordering.
 
+o	You are only permitted to use rcu_dereference on pointer values.
+	The compiler simply knows too much about integral values to
+	trust it to carry dependencies through integer operations.
+	There are a very few exceptions, namely that you can temporarily
+	cast the pointer to uintptr_t in order to:
+
+	o	Set bits and clear bits down in the must-be-zero low-order
+		bits of that pointer.  This clearly means that the pointer
+		must have alignment constraints, for example, this does
+		-not- work in general for char* pointers.
+
+	o	XOR bits to translate pointers, as is done in some
+		classic buddy-allocator algorithms.
+
+	It is important to cast the value back to pointer before
+	doing much of anything else with it.
+
 o	Avoid cancellation when using the "+" and "-" infix arithmetic
 	operators.  For example, for a given variable "x", avoid
-	"(x-x)".  There are similar arithmetic pitfalls from other
-	arithmetic operators, such as "(x*0)", "(x/(x+1))" or "(x%1)".
-	The compiler is within its rights to substitute zero for all of
-	these expressions, so that subsequent accesses no longer depend
-	on the rcu_dereference(), again possibly resulting in bugs due
-	to misordering.
+	"(x-(uintptr_t)x)" for char* pointers.	The compiler is within its
+	rights to substitute zero for this sort of expression, so that
+	subsequent accesses no longer depend on the rcu_dereference(),
+	again possibly resulting in bugs due to misordering.
 
 	Of course, if "p" is a pointer from rcu_dereference(), and "a"
 	and "b" are integers that happen to be equal, the expression
 	"p+a-b" is safe because its value still necessarily depends on
 	the rcu_dereference(), thus maintaining proper ordering.
 
-o	Avoid all-zero operands to the bitwise "&" operator, and
-	similarly avoid all-ones operands to the bitwise "|" operator.
-	If the compiler is able to deduce the value of such operands,
-	it is within its rights to substitute the corresponding constant
-	for the bitwise operation.  Once again, this causes subsequent
-	accesses to no longer depend on the rcu_dereference(), causing
-	bugs due to misordering.
-
-	Please note that single-bit operands to bitwise "&" can also
-	be dangerous.  At this point, the compiler knows that the
-	resulting value can only take on one of two possible values.
-	Therefore, a very small amount of additional information will
-	allow the compiler to deduce the exact value, which again can
-	result in misordering.
-
 o	If you are using RCU to protect JITed functions, so that the
 	"()" function-invocation operator is applied to a value obtained
 	(directly or indirectly) from rcu_dereference(), you may need to
@@ -61,25 +61,6 @@ o	If you are using RCU to protect JITed functions, so that the
 	This issue arises on some systems when a newly JITed function is
 	using the same memory that was used by an earlier JITed function.
 
-o	Do not use the results from the boolean "&&" and "||" when
-	dereferencing.	For example, the following (rather improbable)
-	code is buggy:
-
-		int *p;
-		int *q;
-
-		...
-
-		p = rcu_dereference(gp)
-		q = &global_q;
-		q += p != &oom_p1 && p != &oom_p2;
-		r1 = *q;  /* BUGGY!!! */
-
-	The reason this is buggy is that "&&" and "||" are often compiled
-	using branches.  While weak-memory machines such as ARM or PowerPC
-	do order stores after such branches, they can speculate loads,
-	which can result in misordering bugs.
-
 o	Do not use the results from relational operators ("==", "!=",
 	">", ">=", "<", or "<=") when dereferencing.  For example,
 	the following (quite strange) code is buggy:
-- 
2.5.2

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