cfq-iosched: fix race between exiting queue and exiting task

Original patch from Nikanth Karthikesan <knikanth@suse.de>

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.

Fix this by rechecking the cfq_io_context queue key inside the queue lock
again, and not calling into __cfq_exit_single_io_context() if somebody
beat us to it.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ee8a90c..e8525fa 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1314,7 +1314,15 @@ 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);
+
+               /*
+                * Ensure we get a fresh copy of the ->key to prevent
+                * race between exiting task and queue
+                */
+               smp_read_barrier_depends();
+               if (cic->key)
+                       __cfq_exit_single_io_context(cfqd, cic);
+
                spin_unlock_irqrestore(q->queue_lock, flags);
        }
 }