1 files changed, 24 insertions, 7 deletions

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 607f852b4d04..8d0f35debf35 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -268,6 +268,15 @@ bool try_wait_for_completion(struct completion *x)
         unsigned long flags;
         int ret = 1;
 
+        /*
+         * Since x->done will need to be locked only
+         * in the non-blocking case, we check x->done
+         * first without taking the lock so we can
+         * return early in the blocking case.
+         */
+        if (!READ_ONCE(x->done))
+                return 0;
+
         spin_lock_irqsave(&x->wait.lock, flags);
         if (!x->done)
                 ret = 0;
@@ -288,13 +297,21 @@ EXPORT_SYMBOL(try_wait_for_completion);
  */
 bool completion_done(struct completion *x)
 {
-        unsigned long flags;
-        int ret = 1;
+        if (!READ_ONCE(x->done))
+                return false;
 
-        spin_lock_irqsave(&x->wait.lock, flags);
-        if (!x->done)
-                ret = 0;
-        spin_unlock_irqrestore(&x->wait.lock, flags);
-        return ret;
+        /*
+         * 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);
+        return true;
 }
 EXPORT_SYMBOL(completion_done);