linux-kernel - [PATCH v2 04/13] rwsem: simplify rwsem_down_read

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Message-Id: <1363344869-15732-5-git-send-email-walken@google.com>
Date:	Fri, 15 Mar 2013 03:54:20 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Alex Shi <alex.shi@...el.com>, Ingo Molnar <mingo@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Dave Chinner <david@...morbit.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 04/13] rwsem: simplify rwsem_down_read_failed

When trying to acquire a read lock, the RWSEM_ACTIVE_READ_BIAS adjustment
doesn't cause other readers to block, so we never have to worry about waking
them back after canceling this adjustment in rwsem_down_read_failed().

We also never want to steal the lock in rwsem_down_read_failed(), so we
don't have to grab the wait_lock either.

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

---
 lib/rwsem.c | 22 ++--------------------
 1 file changed, 2 insertions(+), 20 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index fb658af1c12c..66f307e90761 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -182,7 +182,6 @@ try_again_write:
  */
 struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 {
-	enum rwsem_waiter_type type = RWSEM_WAITING_FOR_READ;
 	signed long adjustment = -RWSEM_ACTIVE_READ_BIAS;
 	struct rwsem_waiter waiter;
 	struct task_struct *tsk = current;
@@ -190,7 +189,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 
 	/* set up my own style of waitqueue */
 	waiter.task = tsk;
-	waiter.type = type;
+	waiter.type = RWSEM_WAITING_FOR_READ;
 	get_task_struct(tsk);
 
 	raw_spin_lock_irq(&sem->wait_lock);
@@ -201,17 +200,9 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	/* we're now waiting on the lock, but no longer actively locking */
 	count = rwsem_atomic_update(adjustment, sem);
 
-	/* If there are no active locks, wake the front queued process(es) up.
-	 *
-	 * Alternatively, if we're called from a failed down_write(), there
-	 * were already threads queued before us and there are no active
-	 * writers, the lock must be read owned; so we try to wake any read
-	 * locks that were queued ahead of us. */
+	/* If there are no active locks, wake the front queued process(es). */
 	if (count == RWSEM_WAITING_BIAS)
 		sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
-	else if (count > RWSEM_WAITING_BIAS &&
-		 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
-		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
 
 	raw_spin_unlock_irq(&sem->wait_lock);
 
@@ -220,15 +211,6 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
 		if (!waiter.task)
 			break;
-
-		raw_spin_lock_irq(&sem->wait_lock);
-		/* Try to get the writer sem, may steal from the head writer: */
-		if (type == RWSEM_WAITING_FOR_WRITE)
-			if (try_get_writer_sem(sem, &waiter)) {
-				raw_spin_unlock_irq(&sem->wait_lock);
-				return sem;
-			}
-		raw_spin_unlock_irq(&sem->wait_lock);
 		schedule();
 	}
 
-- 
1.8.1.3
--
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