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From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Jason Low <jason.low2@hp.com>,
	Michel Lespinasse <walken@google.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	linux-kernel@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
	Davidlohr Bueso <dbueso@suse.de>
Subject: [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks
Date: Sun, 25 Jan 2015 23:36:05 -0800	[thread overview]
Message-ID: <1422257769-14083-3-git-send-email-dave@stgolabs.net> (raw)
In-Reply-To: <1422257769-14083-1-git-send-email-dave@stgolabs.net>

The need for the smp_mb in __rwsem_do_wake should be
properly documented. Applies to both xadd and spinlock
variants.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/locking/rwsem-spinlock.c | 5 +++++
 kernel/locking/rwsem-xadd.c     | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c
index 2555ae1..54f7a17 100644
--- a/kernel/locking/rwsem-spinlock.c
+++ b/kernel/locking/rwsem-spinlock.c
@@ -85,6 +85,11 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 
 		list_del(&waiter->list);
 		tsk = waiter->task;
+		/*
+		 * Ensure that all cores see the read before
+		 * setting it to the waiter task to nil, as that
+		 * grants the read lock to the next task.
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2f7cc40..149e8c7 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -186,6 +186,11 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 		waiter = list_entry(next, struct rwsem_waiter, list);
 		next = waiter->list.next;
 		tsk = waiter->task;
+		/*
+		 * Ensure that all cores see the read before
+		 * setting it to the waiter task to nil, as that
+		 * grants the read lock to the next task.
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);
-- 
2.1.2


  parent reply	other threads:[~2015-01-26  7:38 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-26  7:36 [PATCH -tip 0/6] rwsem: Fine tuning Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 1/6] locking/rwsem: Use task->state helpers Davidlohr Bueso
2015-02-04 14:38   ` [tip:locking/core] " tip-bot for Davidlohr Bueso
2015-01-26  7:36 ` Davidlohr Bueso [this message]
2015-01-27 17:07   ` [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks Peter Zijlstra
2015-01-27 20:30     ` Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 3/6] locking/rwsem: Set lock ownership ASAP Davidlohr Bueso
2015-01-27 17:10   ` Peter Zijlstra
2015-01-27 19:18     ` Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 4/6] locking/rwsem: Avoid deceiving lock spinners Davidlohr Bueso
2015-01-27 17:23   ` Jason Low
2015-01-28  3:54     ` Davidlohr Bueso
2015-01-28 17:01       ` Tim Chen
2015-01-28 21:03       ` Jason Low
2015-01-29  1:10         ` Davidlohr Bueso
2015-01-29 20:13           ` Jason Low
2015-01-29 20:18             ` Jason Low
2015-01-29 23:15               ` Davidlohr Bueso
2015-01-30  1:52                 ` Refactoring mutex spin on owner code Jason Low
2015-01-30  7:14                   ` Davidlohr Bueso
2015-01-30  7:51                     ` Peter Zijlstra
2015-01-26  7:36 ` [PATCH 5/6] locking/rwsem: Optimize slowpath/sleeping Davidlohr Bueso
2015-01-27 17:34   ` Peter Zijlstra
2015-01-27 21:57     ` Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 6/6] locking/rwsem: Check for active lock before bailing on spinning Davidlohr Bueso
2015-01-27 18:11   ` Jason Low

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