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Subject: [PATCH 06 of 11] rwsem contended
Message-Id: <74b873f3ea07012e2fc8.1209740709@duo.random>
In-Reply-To: <patchbomb.1209740703@duo.random>
Date: Fri, 02 May 2008 17:05:09 +0200
From: Andrea Arcangeli <andrea@qumranet.com>
Sender: owner-linux-mm@kvack.org
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <clameter@sgi.com>, Jack Steiner <steiner@sgi.com>, Robin Holt <holt@sgi.com>, Nick Piggin <npiggin@suse.de>, Peter Zijlstra <a.p.zijlstra@chello.nl>, kvm-devel@lists.sourceforge.net, Kanoj Sarcar <kanojsarcar@yahoo.com>, Roland Dreier <rdreier@cisco.com>, Steve Wise <swise@opengridcomputing.com>, linux-kernel@vger.kernel.org, Avi Kivity <avi@qumranet.com>, linux-mm@kvack.org, general@lists.openfabrics.org, Hugh Dickins <hugh@veritas.com>, Rusty Russell <rusty@rustcorp.com.au>, Anthony Liguori <aliguori@us.ibm.com>, Chris Wright <chrisw@redhat.com>, Marcelo Tosatti <marcelo@kvack.org>, Eric Dumazet <dada1@cosmosbay.com>, "Paul E. McKenney" <paulmck@us.ibm.com>
List-ID: <linux-mm.kvack.org>

# HG changeset patch
# User Andrea Arcangeli <andrea@qumranet.com>
# Date 1209740186 -7200
# Node ID 74b873f3ea07012e2fc864f203edf1179865feb1
# Parent  a8ac53b928dfcea0ccb326fb7d71f908f0df85f4
rwsem contended

Add a function to rw_semaphores to check if there are any processes
waiting for the semaphore. Add rwsem_needbreak to sched.h that works
in the same way as spinlock_needbreak().

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>

diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -57,6 +57,8 @@ extern void up_write(struct rw_semaphore
  */
 extern void downgrade_write(struct rw_semaphore *sem);
 
+extern int rwsem_is_contended(struct rw_semaphore *sem);
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 /*
  * nested locking. NOTE: rwsems are not allowed to recurse
diff --git a/include/linux/sched.h b/include/linux/sched.h
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2030,6 +2030,15 @@ static inline int spin_needbreak(spinloc
 #endif
 }
 
+static inline int rwsem_needbreak(struct rw_semaphore *sem)
+{
+#ifdef CONFIG_PREEMPT
+	return rwsem_is_contended(sem);
+#else
+	return 0;
+#endif
+}
+
 /*
  * Reevaluate whether the task has signals pending delivery.
  * Wake the task if so.
diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
--- a/lib/rwsem-spinlock.c
+++ b/lib/rwsem-spinlock.c
@@ -305,6 +305,18 @@ void __downgrade_write(struct rw_semapho
 	spin_unlock_irqrestore(&sem->wait_lock, flags);
 }
 
+int rwsem_is_contended(struct rw_semaphore *sem)
+{
+	/*
+	 * Racy check for an empty list. False positives or negatives
+	 * would be okay. False positive may cause a useless dropping of
+	 * locks. False negatives may cause locks to be held a bit
+	 * longer until the next check.
+	 */
+	return !list_empty(&sem->wait_list);
+}
+
+EXPORT_SYMBOL(rwsem_is_contended);
 EXPORT_SYMBOL(__init_rwsem);
 EXPORT_SYMBOL(__down_read);
 EXPORT_SYMBOL(__down_read_trylock);
diff --git a/lib/rwsem.c b/lib/rwsem.c
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -251,6 +251,18 @@ asmregparm struct rw_semaphore *rwsem_do
 	return sem;
 }
 
+int rwsem_is_contended(struct rw_semaphore *sem)
+{
+	/*
+	 * Racy check for an empty list. False positives or negatives
+	 * would be okay. False positive may cause a useless dropping of
+	 * locks. False negatives may cause locks to be held a bit
+	 * longer until the next check.
+	 */
+	return !list_empty(&sem->wait_list);
+}
+
+EXPORT_SYMBOL(rwsem_is_contended);
 EXPORT_SYMBOL(rwsem_down_read_failed);
 EXPORT_SYMBOL(rwsem_down_write_failed);
 EXPORT_SYMBOL(rwsem_wake);

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