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SubjectRE: [PATCH] mtd: fix: avoid race condition when accessing mtd->usecount
Date
> -----Original Message-----
> From: ext Brian Norris [mailto:computersforpeace@gmail.com]
> Sent: Friday, May 08, 2015 2:27 AM
> To: Cantavenera, Giuseppe (EXT-Other - DE/Ulm)
> Cc: linux-mtd@lists.infradead.org; Restelli, Lorenzo (EXT-Other -
> DE/Ulm); dwmw2@infradead.org; stable@vger.kernel.org; Sverdlin,
> Alexander (Nokia - DE/Ulm); zhangxingcai; fengfuqiu@huawei.com;
> tanhaijun@huawei.com; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH] mtd: fix: avoid race condition when accessing mtd-
> >usecount
>
> On Thu, May 07, 2015 at 05:17:45PM -0700, Brian Norris wrote:
> > 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.
>

Hello,
Thanks for your comments and for pointing this out.
Definitely yes.. we shouldn't change del_mtd_blktrans_dev().

> As an alternative to your patch, how about the following?

I think it's the right way to go now.

Thanks!
Giuseppe



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