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SubjectRe: [bug, 2.6.26-rc4/rc5] sporadic bootup crashes in blk_lookup_devt()/prepare_namespace()
On Mon, 9 Jun 2008 09:15:40 -0700 (PDT),
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> And this is all still ignoring the locking issue, of course. It would be
> trivial to just remove the block_class_lock, and change
>
> mutex_[un]lock(&block_class_lock);
>
> into
>
> down|up(&block_class.sem);
>
> except for _one_ case, which is
>
> bdev_map = kobj_map_init(base_probe, &block_class_lock);
>
> which really wants a mutex, not a sempahore.
>
> So to fix that, we'd need to make the class->sem be a mutex, and pass that
> in. Which is probably a good change too, but makes the whole thing much
> bigger.

The driver core changes in -next convert class->sem to
class->p->class_mutex, which makes it non-accessible to drivers.
Most of the locking is easily done through converting to the class
iterator functions, but there are some cases where this is not going to
work:

- The {register,unregister}_blkdev() functions, which don't directly
involve the class.
- The iterators for /proc/partitions, which take the lock in
part_start() and give it up again in part_stop().
Maybe we need a possibilty for a driver to lock a class from outside?


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