linux-kernel - [PATCH v9 4/5] qrwlock: Use smp_store_release() in write

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Date:	Tue, 14 Jan 2014 23:44:06 -0500
From:	Waiman Long <Waiman.Long@...com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>
Cc:	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	"Aswin Chandramouleeswaran\"" <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Waiman Long <Waiman.Long@...com>
Subject: [PATCH v9 4/5] qrwlock: Use smp_store_release() in write_unlock()

This patch modifies the queue_write_unlock() function to use the new
smp_store_release() function (currently in tip). It also removes the
temporary implementation of smp_load_acquire() and smp_store_release()
function in qrwlock.c.

This patch will use atomic subtraction instead if the writer field is
not atomic.

Signed-off-by: Waiman Long <Waiman.Long@...com>
---
 include/asm-generic/qrwlock.h |   10 ++++++----
 kernel/locking/qrwlock.c      |   34 ----------------------------------
 2 files changed, 6 insertions(+), 38 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 5abb6ca..68f488b 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -181,11 +181,13 @@ static inline void queue_read_unlock(struct qrwlock *lock)
 static inline void queue_write_unlock(struct qrwlock *lock)
 {
 	/*
-	 * Make sure that none of the critical section will be leaked out.
+	 * If the writer field is atomic, it can be cleared directly.
+	 * Otherwise, an atomic subtraction will be used to clear it.
 	 */
-	smp_mb__before_clear_bit();
-	ACCESS_ONCE(lock->cnts.writer) = 0;
-	smp_mb__after_clear_bit();
+	if (__native_word(lock->cnts.writer))
+		smp_store_release(&lock->cnts.writer, 0);
+	else
+		atomic_sub(_QW_LOCKED, &lock->cnts.rwa);
 }
 
 /*
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 053be4d..2727188 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -47,40 +47,6 @@
 # define arch_mutex_cpu_relax() cpu_relax()
 #endif
 
-#ifndef smp_load_acquire
-# ifdef CONFIG_X86
-#   define smp_load_acquire(p)				\
-	({						\
-		typeof(*p) ___p1 = ACCESS_ONCE(*p);	\
-		barrier();				\
-		___p1;					\
-	})
-# else
-#   define smp_load_acquire(p)				\
-	({						\
-		typeof(*p) ___p1 = ACCESS_ONCE(*p);	\
-		smp_mb();				\
-		___p1;					\
-	})
-# endif
-#endif
-
-#ifndef smp_store_release
-# ifdef CONFIG_X86
-#   define smp_store_release(p, v)			\
-	do {						\
-		barrier();				\
-		ACCESS_ONCE(*p) = v;			\
-	} while (0)
-# else
-#   define smp_store_release(p, v)			\
-	do {						\
-		smp_mb();				\
-		ACCESS_ONCE(*p) = v;			\
-	} while (0)
-# endif
-#endif
-
 /*
  * If an xadd (exchange-add) macro isn't available, simulate one with
  * the atomic_add_return() function.
-- 
1.7.1

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