cfq-iosched: fix use-after-free of cfqq
With the changes in life time management between the cfq IO contexts and the cfq queues, we now risk having cfqd->active_queue being freed when cfq_slice_expired() is being called. cfq_preempt_queue() caches this queue and uses it after calling said function, causing a use-after-free condition. This triggers the following oops, when cfqq_type() attempts to dereference it: BUG: unable to handle kernel paging request at ffff8800746c4f0c IP: [<ffffffff81266d59>] cfqq_type+0xb/0x20 PGD 18d4063 PUD 1fe15067 PMD 1ffb9067 PTE 80000000746c4160 Oops: 0000 [#1] SMP DEBUG_PAGEALLOC CPU 3 Modules linked in: Pid: 1, comm: init Not tainted 3.2.0-josef+ #367 Bochs Bochs RIP: 0010:[<ffffffff81266d59>] [<ffffffff81266d59>] cfqq_type+0xb/0x20 RSP: 0018:ffff880079c11778 EFLAGS: 00010046 RAX: 0000000000000000 RBX: ffff880076f3df08 RCX: 0000000000000000 RDX: 0000000000000006 RSI: ffff880074271888 RDI: ffff8800746c4f08 RBP: ffff880079c11778 R08: 0000000000000078 R09: 0000000000000001...
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b3c9dd18
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