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Subject[PATCH tip/core/rcu 9/9] rcu: Don't return a value from rcu_assign_pointer()
Date
From: Andrea Parri <andrea.parri@amarulasolutions.com>

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>
---
Documentation/RCU/whatisRCU.txt | 8 ++++----
include/linux/rcupdate.h | 5 ++---
tools/include/linux/rcu.h | 4 ++--
tools/testing/radix-tree/linux/rcupdate.h | 2 +-
4 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 981651a8b65d..7e1a8721637a 100644
--- 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 a8ed624da555..0c9b92799abc 100644
--- 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 7d02527e5bce..9554d3fa54f3 100644
--- 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 fd280b070fdb..fed468fb0c78 100644
--- 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
--
2.17.1
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