linux-kernel - [PATCH RFC tip/core/rcu 9/9] rcu: Change _wait_rcu

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Date:	Fri, 28 Aug 2015 20:36:01 -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,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 9/9] rcu: Change _wait_rcu_gp() to work around GCC bug 67055

From: Oleg Nesterov <oleg@...hat.com>

Code like this in inline functions confuses some recent versions of gcc:

	const int n = const-expr;
	whatever_t array[n];

For more details, see:

	https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67055#c13

This compiler bug results in the following failure after 114b7fd4b (rcu:
Create rcu_sync infrastructure):

	In file included from include/linux/rcupdate.h:429:0,
			  from include/linux/rcu_sync.h:5,
			  from kernel/rcu/sync.c:1:
	include/linux/rcutiny.h: In function 'rcu_barrier_sched':
	include/linux/rcutiny.h:55:20: internal compiler error: Segmentation fault
	  static inline void rcu_barrier_sched(void)

This commit therefore eliminates the constant local variable in favor of
direct use of the expression.

Reported-and-tested-by: Mark Salter <msalter@...hat.com>
Reported-by: Guenter Roeck <linux@...ck-us.net>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/rcupdate.h | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index ff476515f716..581abf848566 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -230,12 +230,11 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 		   struct rcu_synchronize *rs_array);
 
 #define _wait_rcu_gp(checktiny, ...) \
-do { \
-	call_rcu_func_t __crcu_array[] = { __VA_ARGS__ }; \
-	const int __n = ARRAY_SIZE(__crcu_array); \
-	struct rcu_synchronize __rs_array[__n]; \
-	\
-	__wait_rcu_gp(checktiny, __n, __crcu_array, __rs_array); \
+do {									\
+	call_rcu_func_t __crcu_array[] = { __VA_ARGS__ };		\
+	struct rcu_synchronize __rs_array[ARRAY_SIZE(__crcu_array)];	\
+	__wait_rcu_gp(checktiny, ARRAY_SIZE(__crcu_array),		\
+			__crcu_array, __rs_array);			\
 } while (0)
 
 #define wait_rcu_gp(...) _wait_rcu_gp(false, __VA_ARGS__)
-- 
1.8.1.5

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