From 62c1fe9d9f0a676fce89185b1513f0e5f473c72c Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Mon, 15 Dec 2008 21:19:25 +0100 Subject: [PATCH] cfq-iosched: fix race between exiting queue and exiting task Original patch from Nikanth Karthikesan 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 --- block/cfq-iosched.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index ee8a90c7c46c..e8525fa72823 100644 --- 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); } }