Re: [PATCH 1/4] dax: Fix dax_mapping_release() use after free

[Ira Weiny <ira.weiny@intel.com>]

Dan Williams wrote:
> A CONFIG_DEBUG_KOBJECT_RELEASE test of removing a device-dax region
> provider (like modprobe -r dax_hmem) yields:
> 
>  kobject: 'mapping0' (ffff93eb460e8800): kobject_release, parent 0000000000000000 (delayed 2000)
>  [..]
>  DEBUG_LOCKS_WARN_ON(1)
>  WARNING: CPU: 23 PID: 282 at kernel/locking/lockdep.c:232 __lock_acquire+0x9fc/0x2260
>  [..]
>  RIP: 0010:__lock_acquire+0x9fc/0x2260
>  [..]
>  Call Trace:
>   <TASK>
>  [..]
>   lock_acquire+0xd4/0x2c0
>   ? ida_free+0x62/0x130
>   _raw_spin_lock_irqsave+0x47/0x70
>   ? ida_free+0x62/0x130
>   ida_free+0x62/0x130
>   dax_mapping_release+0x1f/0x30
>   device_release+0x36/0x90
>   kobject_delayed_cleanup+0x46/0x150
> 
> Due to attempting ida_free() on an ida object that has already been
> freed. Devices typically only hold a reference on their parent while
> registered. If a child needs a parent object to complete its release it
> needs to hold a reference that it drops from its release callback.
> Arrange for a dax_mapping to pin its parent dev_dax instance until
> dax_mapping_release().
> 
> Fixes: 0b07ce872a9e ("device-dax: introduce 'mapping' devices")

Reviewed-by: Ira Weiny <ira.weiny@intel.com>