rcu: Don't return a value from rcu_assign_pointer()

Quoting Paul [1]:

  "Given that a quick (and perhaps error-prone) search of the uses
   of rcu_assign_pointer() in v5.1 didn't find a single use of the
   return value, let's please instead change the documentation and
   implementation to eliminate the return value."

[1] https://lkml.kernel.org/r/20190523135013.GL28207@linux.ibm.com

Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: rcu@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 981651a8b65d206bb073f40c982d3fc5314c4d70..7e1a8721637ab764c10106b8e947b5fce9c4358d 100644 (file)
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -212,7 +212,7 @@ synchronize_rcu()
 
 rcu_assign_pointer()
 
-       typeof(p) rcu_assign_pointer(p, typeof(p) v);
+       void rcu_assign_pointer(p, typeof(p) v);
 
        Yes, rcu_assign_pointer() -is- implemented as a macro, though it
        would be cool to be able to declare a function in this manner.
@@ -220,9 +220,9 @@ rcu_assign_pointer()
 
        The updater uses this function to assign a new value to an
        RCU-protected pointer, in order to safely communicate the change
-       in value from the updater to the reader.  This function returns
-       the new value, and also executes any memory-barrier instructions
-       required for a given CPU architecture.
+       in value from the updater to the reader.  This macro does not
+       evaluate to an rvalue, but it does execute any memory-barrier
+       instructions required for a given CPU architecture.
 
        Perhaps just as important, it serves to document (1) which
        pointers are protected by RCU and (2) the point at which a

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index a8ed624da555640084f76683db5ee60f9c397334..0c9b92799abc74787558237d589235d01c4b08b5 100644 (file)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -367,7 +367,7 @@ static inline void rcu_preempt_sleep_check(void) { }
  * other macros that it invokes.
  */
 #define rcu_assign_pointer(p, v)                                             \
-({                                                                           \
+do {                                                                         \
        uintptr_t _r_a_p__v = (uintptr_t)(v);                                 \
        rcu_check_sparse(p, __rcu);                                           \
                                                                              \
@@ -375,8 +375,7 @@ static inline void rcu_preempt_sleep_check(void) { }
                WRITE_ONCE((p), (typeof(p))(_r_a_p__v));                      \
        else                                                                  \
                smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
-       _r_a_p__v;                                                            \
-})
+} while (0)
 
 /**
  * rcu_swap_protected() - swap an RCU and a regular pointer

diff --git a/tools/include/linux/rcu.h b/tools/include/linux/rcu.h
index 7d02527e5bcea4bd577c8f7062fc0c92a00be218..9554d3fa54f3336ea96d30a133338a29e2d71e2e 100644 (file)
--- a/tools/include/linux/rcu.h
+++ b/tools/include/linux/rcu.h
@@ -19,7 +19,7 @@ static inline bool rcu_is_watching(void)
        return false;
 }
 
-#define rcu_assign_pointer(p, v) ((p) = (v))
-#define RCU_INIT_POINTER(p, v) p=(v)
+#define rcu_assign_pointer(p, v)       do { (p) = (v); } while (0)
+#define RCU_INIT_POINTER(p, v) do { (p) = (v); } while (0)
 
 #endif

diff --git a/tools/testing/radix-tree/linux/rcupdate.h b/tools/testing/radix-tree/linux/rcupdate.h
index fd280b070fdb1f03a098b94e2feaf1c3b9f2324f..fed468fb0c78db6f33fb1900c7110ab5f3c19c65 100644 (file)
--- a/tools/testing/radix-tree/linux/rcupdate.h
+++ b/tools/testing/radix-tree/linux/rcupdate.h
@@ -7,6 +7,6 @@
 #define rcu_dereference_raw(p) rcu_dereference(p)
 #define rcu_dereference_protected(p, cond) rcu_dereference(p)
 #define rcu_dereference_check(p, cond) rcu_dereference(p)
-#define RCU_INIT_POINTER(p, v) (p) = (v)
+#define RCU_INIT_POINTER(p, v) do { (p) = (v); } while (0)
 
 #endif