linux-kernel - [PATCH 2/4] rwsem: shorter spinlocked section in rwsem_down_failed

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Message-Id: <1360377937-16297-3-git-send-email-walken@google.com>
Date:	Fri,  8 Feb 2013 18:45:35 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	linux-kernel@...r.kernel.org, anton@...ba.org, hpa@...or.com,
	mingo@...nel.org, arjan@...ux.intel.com, a.p.zijlstra@...llo.nl,
	torvalds@...ux-foundation.org, alex.shi@...el.com,
	yuanhan.liu@...ux.intel.com, dhowells@...hat.com,
	akpm@...ux-foundation.org, tglx@...utronix.de
Subject: [PATCH 2/4] rwsem: shorter spinlocked section in rwsem_down_failed_common()

This change reduces the size of the spinlocked and TASK_UNINTERRUPTIBLE
sections in rwsem_down_failed_common():

- We only need the sem->wait_lock to insert ourselves on the wait_list;
  the waiter node can be prepared outside of the wait_lock.

- The task state only needs to be set to TASK_UNINTERRUPTIBLE immediately
  before testing waiter.task to see if someone woke us; it doesn't need to
  protect the entire function.

Signed-off-by: Michel Lespinasse <walken@...gle.com>

---
 lib/rwsem.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 4a6ff093a433..cd2d803cbbe4 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -180,14 +180,12 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 	struct task_struct *tsk = current;
 	signed long count;
 
-	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
-
 	/* set up my own style of waitqueue */
-	raw_spin_lock_irq(&sem->wait_lock);
 	waiter.task = tsk;
 	waiter.type = type;
 	get_task_struct(tsk);
 
+	raw_spin_lock_irq(&sem->wait_lock);
 	if (list_empty(&sem->wait_list))
 		adjustment += RWSEM_WAITING_BIAS;
 	list_add_tail(&waiter.list, &sem->wait_list);
@@ -210,11 +208,11 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 	raw_spin_unlock_irq(&sem->wait_lock);
 
 	/* wait to be given the lock */
-	for (;;) {
+	while (true) {
+		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 		if (!waiter.task)
 			break;
 		schedule();
-		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 	}
 
 	tsk->state = TASK_RUNNING;
-- 
1.8.1
--
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