linux-kernel - Re: [PATCH] fix race condition between device_del and device

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Date:	Mon, 30 Mar 2015 12:04:11 +0200
From:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To:	EunTaik Lee <eun.taik.lee@...sung.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fix race condition between device_del and device_add

On Mon, Mar 30, 2015 at 09:06:43AM +0000, EunTaik Lee wrote:
> There is a possible race condition when a device
> is added while another device with the same parent
> , with ref count of one, is deleted.
> 
>  CPU0    CPU1
> device_add()   device_del()
>   get_device_parent()     put_device(parent);
>     kobj = kobject_get(k);     kobject_put()
>           kref_put()
>        refcount=0
>       refcount is 0
>       WARNS it was 0 but
>       return kobj            frees kobj
> uses the freed parent kobj
> 
> The race condition exists because kref_put() and the
> release function is not atomic.
> Using kobject_get_unless_zero() instead of
> kobject_get() in get_device_parent() will prevent
> this race condition.

The bus that the device is on should prevent this, why isn't that
working for you?  What type of device/bus do you see this problem on?

> Signed-off-by: eun.taik.lee <eun.taik.lee@...sung.com>

We need a "real" name here, I don't think your name has '.' in it,
right?

> ---
>  drivers/base/core.c     |    2 +-
>  include/linux/kobject.h |    2 ++
>  lib/kobject.c           |    2 +-
>  3 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 07304a3..ec2f211 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -761,7 +761,7 @@ static struct kobject *get_device_parent(struct device
> *dev,
>    spin_lock(&dev->class->p->glue_dirs.list_lock);
>    list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
>     if (k->parent == parent_kobj) {
> -    kobj = kobject_get(k);
> +    kobj = kobject_get_unless_zero(k);
>      break;

Your email client turned all tabs to spaces and made the patch unable to
be applied.

thanks,

greg k-h
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