Use of spinlock after free with CFQ scheduler

[Thomas Petazzoni <thomas.petazzoni@enix.org>]

Hi,

While developing a block device driver, we stumbled upon the kernel
panic reported at
http://www.ussg.iu.edu/hypermail/linux/kernel/0512.3/0297.html.
According to the mail and your answer, it seems that the CFQ scheduler
uses the queue lock after blk_cleanup_queue(). At this time, the
spinlock might have been freed. I can confirm that the bug doesn't
appear with other I/O schedulers.

However, the proposed fix for "ub" looks quite strange to me. It uses
a static array of spinlocks, so that they remain in memory after
blk_cleanup_queue(). However, "ub" can be compiled as a module, so I
don't see what prevent the use of the queue spinlocks by the CFQ
scheduler once the module has been unloaded. I do not understand how the
provided patch correctly fixes the bug.

The bug was reported on a pre-2.6.15 kernel, but we're still seeing
this bug with a 2.6.16 FedoraCore-hacked kernel.

To me, the bug seems to be in the CFQ scheduler itself, isn't it ?
Maybe we should use the internal queue lock (by passing NULL as the
lock parameter to the blk_init_queue() call), and then modify the CFQ
scheduler so that it correctly increments/decrements the queue->refcnt ?

What do you think about it ?

Thanks!

Thomas
-- 
Thomas Petazzoni - thomas.petazzoni@enix.org
http://{thomas,sos,kos}.enix.org - http://www.toulibre.org
http://www.{livret,agenda}dulibre.org