Re: [PATCH] mm: memory_hotplug: use put_device() if device_register fail

[arvindY]

On Friday 27 April 2018 08:26 PM, Michal Hocko wrote:
> On Thu 26-04-18 21:12:09, Arvind Yadav wrote:
> > if device_register() returned an error. Always use put_device()
> > to give up the initialized reference and release allocated memory.
> Is this patch correct? The docummentation says
>   * NOTE: _Never_ directly free @dev after calling this function, even
>   * if it returned an error! Always use put_device() to give up your
>   * reference instead.
>
> but we do not have _our_ reference in this path AFAICS. Maybe this is
> just a documentation issue? How have you tested this change btw.?
The document is correct. Here device_register() will initialize object by
making reference count as 1 and also increment reference count for device.

device_register() {
   device_initialize()->kobject_init()->kref_init() - initialize 
object( reference count = 1).
   device_add()->get_device() - increment reference count for device.
}

If device_register() will fail then we have to release the object by making
reference count 0. So we need to call put_object() which will release the
object and other resources like memory etc. so long as the reference
count is nonzero, the object continue to exist in memory.

I have not tested this peace of code. But I have tested other code which
is using Kboject.

~arvind
> > Signed-off-by: Arvind Yadav <arvind.yadav.cs@xxxxxxxxx>
> > ---
> >   drivers/base/memory.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> > index bffe861..f5e5601 100644
> > --- a/drivers/base/memory.c
> > +++ b/drivers/base/memory.c
> > @@ -649,13 +649,19 @@ static const struct attribute_group *memory_memblk_attr_groups[] = {
> >   static
> >   int register_memory(struct memory_block *memory)
> >   {
> > +	int ret;
> > +
> >   	memory->dev.bus = &memory_subsys;
> >   	memory->dev.id = memory->start_section_nr / sections_per_block;
> >   	memory->dev.release = memory_block_release;
> >   	memory->dev.groups = memory_memblk_attr_groups;
> >   	memory->dev.offline = memory->state == MEM_OFFLINE;
> >   
> > -	return device_register(&memory->dev);
> > +	ret = device_register(&memory->dev);
> > +	if (ret)
> > +		put_device(&memory->dev);
> > +
> > +	return ret;
> >   }
> >   
> >   static int init_memory_block(struct memory_block **memory,
> > --
> > 2.7.4