linux-kernel - Re: [PATCH] mm: memory_hotplug: use put_device() if device

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Date:   Sat, 28 Apr 2018 11:05:44 +0530
From:   arvindY <arvind.yadav.cs@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memory_hotplug: use put_device() if device_register
 fail



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@...il.com>
>> ---
>>   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