linux-kernel - [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks

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Message-Id: <1422257769-14083-3-git-send-email-dave@stgolabs.net>
Date:	Sun, 25 Jan 2015 23:36:05 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks

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@...e.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

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