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Subject[PATCH] Exiting queue and task might race to free cic
Date
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);
}
}



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