From: Davidlohr Bueso <dave@stgolabs.net>
Date: Fri, 30 Jan 2015 09:14:24 +0000 (-0800)
Subject: locking/rwsem: Document barrier need when waking tasks
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=49e4b2bcf7b812e985e65b6c8a0255b1520a6e7e;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git

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@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/1422609267-15102-3-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---

diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c
index 2555ae15ec14..3a5048572065 100644
--- a/kernel/locking/rwsem-spinlock.c
+++ b/kernel/locking/rwsem-spinlock.c
@@ -85,6 +85,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 
 		list_del(&waiter->list);
 		tsk = waiter->task;
+		/*
+		 * Make sure we do not wakeup the next reader before
+		 * setting the nil condition to grant the next reader;
+		 * otherwise we could miss the wakeup on the other
+		 * side and end up sleeping again. See the pairing
+		 * in rwsem_down_read_failed().
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2f7cc4076f50..82aba467564a 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -186,6 +186,13 @@ __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;
+		/*
+		 * Make sure we do not wakeup the next reader before
+		 * setting the nil condition to grant the next reader;
+		 * otherwise we could miss the wakeup on the other
+		 * side and end up sleeping again. See the pairing
+		 * in rwsem_down_read_failed().
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);