linux-kernel - [tip:locking/core] locking/spinlocks: Always evaluate the second argument of spin_lock

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Date:	Wed, 13 Aug 2014 03:56:40 -0700
From:	tip-bot for Bart Van Assche <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	davem@...emloft.net, paulmck@...ux.vnet.ibm.com,
	bvanassche@....org, akpm@...ux-foundation.org, oleg@...hat.com,
	tglx@...utronix.de, rientjes@...gle.com
Subject: [tip:locking/core] locking/spinlocks:
  Always evaluate the second argument of spin_lock_nested()

Commit-ID:  4999201a59ef555f9105d2bb2459ed895627f7aa
Gitweb:     http://git.kernel.org/tip/4999201a59ef555f9105d2bb2459ed895627f7aa
Author:     Bart Van Assche <bvanassche@....org>
AuthorDate: Fri, 8 Aug 2014 12:35:36 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 13 Aug 2014 10:32:38 +0200

locking/spinlocks: Always evaluate the second argument of spin_lock_nested()

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>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: David S. Miller <davem@...emloft.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/53E4A7F8.1040700@acm.org
Signed-off-by: Ingo Molnar <mingo@...nel.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
 
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