[PATCH v5 tip/core/rcu 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair

[Paul E. McKenney]

There is no agreed-upon definition of spin_unlock_wait()'s semantics,
and it appears that all callers could do just as well with a lock/unlock
pair.  This commit therefore replaces the spin_unlock_wait() call in
completion_done() with spin_lock() followed immediately by spin_unlock().
This should be safe from a performance perspective because the lock
will be held only the wakeup happens really quickly.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[ paulmck: Updated to use irqsave based on 0day Test Robot feedback. ]

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 13fc5ae9bf2f..c9524d2d9316 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -300,6 +300,8 @@ EXPORT_SYMBOL(try_wait_for_completion);
  */
 bool completion_done(struct completion *x)
 {
+	unsigned long flags;
+
 	if (!READ_ONCE(x->done))
 		return false;
 
@@ -307,14 +309,9 @@ bool completion_done(struct completion *x)
 	 * If ->done, we need to wait for complete() to release ->wait.lock
 	 * otherwise we can end up freeing the completion before complete()
 	 * is done referencing it.
-	 *
-	 * The RMB pairs with complete()'s RELEASE of ->wait.lock and orders
-	 * the loads of ->done and ->wait.lock such that we cannot observe
-	 * the lock before complete() acquires it while observing the ->done
-	 * after it's acquired the lock.
 	 */
-	smp_rmb();
-	spin_unlock_wait(&x->wait.lock);
+	spin_lock_irqsave(&x->wait.lock, flags);
+	spin_unlock_irqrestore(&x->wait.lock, flags);
 	return true;
 }
 EXPORT_SYMBOL(completion_done);