sched/completion: Serialize completion_done() with complete()

Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
reading completion state" was not correct, without lock/unlock the code
like stop_machine_from_inactive_cpu()

	while (!completion_done())
		cpu_relax();

can return before complete() finishes its spin_unlock() which writes to
this memory. And spin_unlock_wait().

While at it, change try_wait_for_completion() to use READ_ONCE().

Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reported-by: Davidlohr Bueso <dave@stgolabs.net>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[ Added a comment with the barrier. ]
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: raghavendra.kt@linux.vnet.ibm.com
Cc: waiman.long@hp.com
Fixes: de30ec47302c ("sched/completion: Remove unnecessary ->wait.lock serialization when reading completion state")
Link: http://lkml.kernel.org/r/20150212195913.GA30430@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 17 insertions, 2 deletions

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 7052d3fd4e7b..8d0f35debf35 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -274,7 +274,7 @@ bool try_wait_for_completion(struct completion *x)
          * first without taking the lock so we can
          * return early in the blocking case.
          */
-        if (!ACCESS_ONCE(x->done))
+        if (!READ_ONCE(x->done))
                 return 0;
 
         spin_lock_irqsave(&x->wait.lock, flags);
@@ -297,6 +297,21 @@ EXPORT_SYMBOL(try_wait_for_completion);
  */
 bool completion_done(struct completion *x)
 {
-        return !!ACCESS_ONCE(x->done);
+        if (!READ_ONCE(x->done))
+                return false;
+
+        /*
+         * 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);