linux-kernel - [PATCH 2/7] locking/rwsem: more aggressive use of optimistic spinning

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Date:	Sun,  3 Aug 2014 22:36:17 -0400
From:	Waiman Long <Waiman.Long@...com>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-doc@...r.kernel.org, Davidlohr Bueso <davidlohr@...com>,
	Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>,
	Waiman Long <Waiman.Long@...com>
Subject: [PATCH 2/7] locking/rwsem: more aggressive use of optimistic spinning

The rwsem_can_spin_on_owner() function currently allows optimistic
spinning only if the owner field is defined and is running. That is
too conservative as it will cause some tasks to miss the opportunity
of doing spinning in case the owner hasn't been able to set the owner
field in time or the lock has just become available.

This patch enables more aggressive use of optimistic spinning by
assuming that the lock is spinnable unless proved otherwise.

Signed-off-by: Waiman Long <Waiman.Long@...com>
---
 kernel/locking/rwsem-xadd.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index d058946..dce22b8 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -285,7 +285,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 {
 	struct task_struct *owner;
-	bool on_cpu = false;
+	bool on_cpu = true;	/* Assume spinnable unless proved not to be */
 
 	if (need_resched())
 		return false;
-- 
1.7.1

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