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 2555ae1..3a50485 100644 (file)
--- 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 2f7cc40..82aba46 100644 (file)
--- 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);