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/[e-mail]

Signed-off-by: Andrea Parri <[e-mail]>
Cc: "Paul E. McKenney" <[e-mail]>
Cc: Josh Triplett <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Lai Jiangshan <[e-mail]>
Cc: Joel Fernandes <[e-mail]>
Cc: [e-mail]
Cc: Peter Zijlstra <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Sasha Levin <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/whatisRCU.txt

···
 
 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.
···
 
 	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

include/linux/rcupdate.h

···
  * 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);					      \
 									      \
···
 		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

tools/include/linux/rcu.h

···
 	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

tools/testing/radix-tree/linux/rcupdate.h

···
 #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