[PATCH] Exiting queue and task might race to free cic

[Nikanth Karthikesan <knikanth@suse.de>]

Hi Jens

Looking at the bug reported here
http://thread.gmane.org/gmane.linux.kernel/722539
it looks like an exiting queue can race with an exiting task.

When a queue exits the queue lock is taken and cfq_exit_queue() would free all 
the cic's associated with the queue.

But when a task exits, cfq_exit_io_context() gets cic one by one and then 
locks the associated queue to call __cfq_exit_single_io_context. It looks like 
between getting a cic from the ioc and locking the queue, the queue might have 
exited on another cpu. Isn't this possible?

If possible, either verifying whether cic->key is still not null or q->flags 
does not have QUEUE_FLAG_DEAD set would fix this.

Thanks
Nikanth Karthikesan

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>

---
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6a062ee..b9b627a 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1318,7 +1318,12 @@ static void cfq_exit_single_io_context(struct 
io_context *ioc,
 		unsigned long flags;
 
 		spin_lock_irqsave(q->queue_lock, flags);
-		__cfq_exit_single_io_context(cfqd, cic);
+		/*
+		 * cic might have been already exited when an exiting task
+		 * races with an exiting queue.
+		 */
+		if (likely(cic->key))
+			__cfq_exit_single_io_context(cfqd, cic);
 		spin_unlock_irqrestore(q->queue_lock, flags);
 	}
 }

Or this would also work

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6a062ee..7a068bd 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1318,7 +1318,11 @@ static void cfq_exit_single_io_context(struct 
io_context *ioc,
 		unsigned long flags;
 
 		spin_lock_irqsave(q->queue_lock, flags);
-		__cfq_exit_single_io_context(cfqd, cic);
+		/*
+		 * Make sure the queue is not dead.
+		 */
+		if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
+			__cfq_exit_single_io_context(cfqd, cic);
 		spin_unlock_irqrestore(q->queue_lock, flags);
 	}
 }