[PATCH] mtd: fix: avoid race condition when accessing mtd->usecount

[Brian Norris]

On Thu, May 07, 2015 at 05:10:12PM -0700, Brian Norris wrote:
> On Tue, Apr 21, 2015 at 12:20:22PM +0200, Giuseppe Cantavenera wrote:
> > @@ -484,7 +486,7 @@ int del_mtd_blktrans_dev(struct mtd_blktrans_dev *old)
> >  	if (old->open) {
> >  		if (old->tr->release)
> >  			old->tr->release(old);
> > -		__put_mtd_device(old->mtd);
> > +		put_mtd_device(old->mtd);
> 
> This looks wrong. See:
[...]
> deregister_mtd_blktrans()
> |_ mutex_lock(&mtd_table_mutex)
> |_ tr->remove_dev() -> inftl_remove_dev()
>    |_ del_mtd_blktrans_dev()
>       |_ put_mtd_device()
>          |_ mutex_lock(&mtd_table_mutex) <--- AA deadlock

What's more, this code in del_mtd_blktrans_dev() makes it obvious that
this hunk is wrong:

int del_mtd_blktrans_dev(struct mtd_blktrans_dev *old)
{
        unsigned long flags;

        if (mutex_trylock(&mtd_table_mutex)) {
                mutex_unlock(&mtd_table_mutex);
                BUG();
        }
	...

So rather than a comment, the code is showing that it's a BUG() to not
be holding mtd_table_mutex already.

Brian