Re: [PATCH] sched/completion: completion_done() should serialize with complete()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote:
> 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().

So I share Davidlohrs concern if we should not simply revert that
change; but given we've now gone over it detail I suppose we should just
keep the optimized version.

I did add a comment to your patch; and queued the below for
sched/urgent.

---
Subject: sched/completion: completion_done() should serialize with complete()
From: Oleg Nesterov <oleg@redhat.com>
Date: Thu, 12 Feb 2015 20:59:13 +0100

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().

Fixes: de30ec47302c ("sched/completion: Remove unnecessary ->wait.lock serialization when reading completion state")
Cc: waiman.long@hp.com
Cc: raghavendra.kt@linux.vnet.ibm.com
Cc: dave@stgolabs.net
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Tested-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Reported-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[peterz: Add a comment with the barrier]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150212195913.GA30430@redhat.com
---
 kernel/sched/completion.c |   19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp
 	 * 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);