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Subject[14/35] cfq-iosched: fix locking around ioc->ioc_data assignment
2.6.33-longterm review patch.  If anyone has any objections, please let us know.

------------------
From: Jens Axboe <jaxboe@fusionio.com>

commit ab4bd22d3cce6977dc039664cc2d052e3147d662 upstream.

Since we are modifying this RCU pointer, we need to hold
the lock protecting it around it.

This fixes a potential reuse and double free of a cfq
io_context structure. The bug has been in CFQ for a long
time, it hit very few people but those it did hit seemed
to see it a lot.

Tracked in RH bugzilla here:

https://bugzilla.redhat.com/show_bug.cgi?id=577968

Credit goes to Paul Bolle for figuring out that the issue
was around the one-hit ioc->ioc_data cache. Thanks to his
hard work the issue is now fixed.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
block/cfq-iosched.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2537,8 +2537,11 @@ static void __cfq_exit_single_io_context
cic->dead_key = (unsigned long) cic->key;
cic->key = NULL;

- if (ioc->ioc_data == cic)
+ if (rcu_dereference(ioc->ioc_data) == cic) {
+ spin_lock(&ioc->lock);
rcu_assign_pointer(ioc->ioc_data, NULL);
+ spin_unlock(&ioc->lock);
+ }

if (cic->cfqq[BLK_RW_ASYNC]) {
cfq_exit_cfqq(cfqd, cic->cfqq[BLK_RW_ASYNC]);



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