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

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

[Brian Norris]

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.

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

BTW, this does still leave a usecount race in
drivers/mtd/maps/vmu-flash.c. But that driver should really be using
mtd->_get_device(), if it actually wants its own refcount.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
---

diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
index 2b0c52870999..df7c6c70757a 100644
--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c
@@ -197,6 +197,7 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode)
 		return -ERESTARTSYS; /* FIXME: busy loop! -arnd*/
 
 	mutex_lock(&dev->lock);
+	mutex_lock(&mtd_table_mutex);
 
 	if (dev->open)
 		goto unlock;
@@ -220,6 +221,7 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode)
 
 unlock:
 	dev->open++;
+	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
 	blktrans_dev_put(dev);
 	return ret;
@@ -230,6 +232,7 @@ error_release:
 error_put:
 	module_put(dev->tr->owner);
 	kref_put(&dev->ref, blktrans_dev_release);
+	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
 	blktrans_dev_put(dev);
 	return ret;
@@ -243,6 +246,7 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode)
 		return;
 
 	mutex_lock(&dev->lock);
+	mutex_lock(&mtd_table_mutex);
 
 	if (--dev->open)
 		goto unlock;
@@ -256,6 +260,7 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode)
 		__put_mtd_device(dev->mtd);
 	}
 unlock:
+	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
 	blktrans_dev_put(dev);
 }

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

[Cantavenera, Giuseppe (EXT-Other - DE/Ulm)]

> -----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

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

[Brian Norris]

On Mon, May 11, 2015 at 07:44:26AM +0000, Cantavenera, Giuseppe (EXT-Other - DE/Ulm) wrote:
> > -----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.

Can I get a 'Tested-by', or at least an 'Acked-by' for the patch? I
tested it, but I don't think I can reproduce your original problem very
easily.

Brian

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

[Cantavenera, Giuseppe (EXT-Other - DE/Ulm)]

> -----Original Message-----
> From: ext Brian Norris [mailto:computersforpeace@gmail.com]
> Sent: Tuesday, May 12, 2015 12:25 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 Mon, May 11, 2015 at 07:44:26AM +0000, Cantavenera, Giuseppe (EXT-
> Other - DE/Ulm) wrote:
> > > -----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:

> Can I get a 'Tested-by', or at least an 'Acked-by' for the patch? I
> tested it, but I don't think I can reproduce your original problem very
> easily.
> 
> Brian

Hello Brian,

We were able to do some long runs last night and as expected
this patch seems to address the issue pretty well.

Thanks,
Giuseppe

Tested-by: Giuseppe Cantavenera <giuseppe.cantavenera.ext@nokia.com>

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

[Brian Norris]

On Tue, May 12, 2015 at 06:38:34AM +0000, Cantavenera, Giuseppe (EXT-Other - DE/Ulm) wrote:
> > -----Original Message-----
> > From: ext Brian Norris [mailto:computersforpeace@gmail.com]
> > Sent: Tuesday, May 12, 2015 12:25 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 Mon, May 11, 2015 at 07:44:26AM +0000, Cantavenera, Giuseppe (EXT-
> > Other - DE/Ulm) wrote:
> > > > -----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:
> 
> > Can I get a 'Tested-by', or at least an 'Acked-by' for the patch? I
> > tested it, but I don't think I can reproduce your original problem very
> > easily.
> > 
> > Brian
> 
> Hello Brian,
> 
> We were able to do some long runs last night and as expected
> this patch seems to address the issue pretty well.
> 
> Thanks,
> Giuseppe
> 
> Tested-by: Giuseppe Cantavenera <giuseppe.cantavenera.ext@nokia.com>

Great, thanks to all of you. I reworked the patch description and
applied this to l2-mtd.git.

Brian

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

[Alexander Sverdlin]

On 08/05/15 02:26, ext Brian Norris wrote:
> As an alternative to your patch, how about the following?
> 
> BTW, this does still leave a usecount race in
> drivers/mtd/maps/vmu-flash.c. But that driver should really be using
> mtd->_get_device(), if it actually wants its own refcount.
> 
> Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Acked-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>

> ---
> 
> diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
> index 2b0c52870999..df7c6c70757a 100644
> --- a/drivers/mtd/mtd_blkdevs.c
> +++ b/drivers/mtd/mtd_blkdevs.c
> @@ -197,6 +197,7 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode)
>  		return -ERESTARTSYS; /* FIXME: busy loop! -arnd*/
>  
>  	mutex_lock(&dev->lock);
> +	mutex_lock(&mtd_table_mutex);
>  
>  	if (dev->open)
>  		goto unlock;
> @@ -220,6 +221,7 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode)
>  
>  unlock:
>  	dev->open++;
> +	mutex_unlock(&mtd_table_mutex);
>  	mutex_unlock(&dev->lock);
>  	blktrans_dev_put(dev);
>  	return ret;
> @@ -230,6 +232,7 @@ error_release:
>  error_put:
>  	module_put(dev->tr->owner);
>  	kref_put(&dev->ref, blktrans_dev_release);
> +	mutex_unlock(&mtd_table_mutex);
>  	mutex_unlock(&dev->lock);
>  	blktrans_dev_put(dev);
>  	return ret;
> @@ -243,6 +246,7 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode)
>  		return;
>  
>  	mutex_lock(&dev->lock);
> +	mutex_lock(&mtd_table_mutex);
>  
>  	if (--dev->open)
>  		goto unlock;
> @@ -256,6 +260,7 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode)
>  		__put_mtd_device(dev->mtd);
>  	}
>  unlock:
> +	mutex_unlock(&mtd_table_mutex);
>  	mutex_unlock(&dev->lock);
>  	blktrans_dev_put(dev);
>  }