linux-kernel - [PATCH v3] spin_lock_nested(): Always evaluate second argument

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Date:	Fri, 08 Aug 2014 12:35:36 +0200
From:	Bart Van Assche <bvanassche@....org>
To:	Ingo Molnar <mingo@...nel.org>,
	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH v3] spin_lock_nested(): Always evaluate second argument

Evaluating a macro argument only if certain configuration options
have been selected is confusing and error-prone. Hence always
evaluate the second argument of spin_lock_nested().

An intentional side effect of this patch is that it avoids that
the following warning is reported for netif_addr_lock_nested()
when building with CONFIG_DEBUG_LOCK_ALLOC=n and with W=1:

include/linux/netdevice.h: In function 'netif_addr_lock_nested':
include/linux/netdevice.h:2865:6: warning: variable 'subclass' set but not used [-Wunused-but-set-variable]
  int subclass = SINGLE_DEPTH_NESTING;
      ^

Signed-off-by: Bart Van Assche <bvanassche@....org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: David S. Miller <davem@...emloft.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>
---
 include/linux/spinlock.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 3f2867f..262ba4e 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -197,7 +197,13 @@ static inline void do_raw_spin_unlock(raw_spinlock_t *lock) __releases(lock)
 		 _raw_spin_lock_nest_lock(lock, &(nest_lock)->dep_map);	\
 	 } while (0)
 #else
-# define raw_spin_lock_nested(lock, subclass)		_raw_spin_lock(lock)
+/*
+ * Always evaluate the 'subclass' argument to avoid that the compiler
+ * warns about set-but-not-used variables when building with
+ * CONFIG_DEBUG_LOCK_ALLOC=n and with W=1.
+ */
+# define raw_spin_lock_nested(lock, subclass)		\
+	_raw_spin_lock(((void)(subclass), (lock)))
 # define raw_spin_lock_nest_lock(lock, nest_lock)	_raw_spin_lock(lock)
 #endif
 
-- 
1.8.4.5

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