Re: [Xen-devel] [PATCH RFC 1/2] drivers/base: export lock_device_hotplug/unlock_device_hotplug

[David Hildenbrand <david@xxxxxxxxxx>]

On 17.08.2018 11:03, Rafael J. Wysocki wrote:
> On Fri, Aug 17, 2018 at 10:56 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 17.08.2018 10:41, Greg Kroah-Hartman wrote:
>>> On Fri, Aug 17, 2018 at 09:59:00AM +0200, David Hildenbrand wrote:
>>>> From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>>>>
>>>> Well require to call add_memory()/add_memory_resource() with
>>>> device_hotplug_lock held, to avoid a lock inversion. Allow external modules
>>>> (e.g. hv_balloon) that make use of add_memory()/add_memory_resource() to
>>>> lock device hotplug.
>>>>
>>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>>>> [modify patch description]
>>>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>>>> ---
>>>>  drivers/base/core.c | 2 ++
>>>>  1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/drivers/base/core.c b/drivers/base/core.c
>>>> index 04bbcd779e11..9010b9e942b5 100644
>>>> --- a/drivers/base/core.c
>>>> +++ b/drivers/base/core.c
>>>> @@ -700,11 +700,13 @@ void lock_device_hotplug(void)
>>>>  {
>>>>      mutex_lock(&device_hotplug_lock);
>>>>  }
>>>> +EXPORT_SYMBOL_GPL(lock_device_hotplug);
>>>>
>>>>  void unlock_device_hotplug(void)
>>>>  {
>>>>      mutex_unlock(&device_hotplug_lock);
>>>>  }
>>>> +EXPORT_SYMBOL_GPL(unlock_device_hotplug);
>>>
>>> If these are going to be "global" symbols, let's properly name them.
>>> device_hotplug_lock/unlock would be better.  But I am _really_ nervous
>>> about letting stuff outside of the driver core mess with this, as people
>>> better know what they are doing.
>>
>> The only "problem" is that we have kernel modules (for paravirtualized
>> devices) that call add_memory(). This is Hyper-V right now, but we might
>> have other ones in the future. Without them we would not have to export
>> it. We might also get kernel modules that want to call remove_memory() -
>> which will require the device_hotplug_lock as of now.
>>
>> What we could do is
>>
>> a) add_memory() -> _add_memory() and don't export it
>> b) add_memory() takes the device_hotplug_lock and calls _add_memory() .
>> We export that one.
>> c) Use add_memory() in external modules only
>>
>> Similar wrapper would be needed e.g. for remove_memory() later on.
> 
> That would be safer IMO, as it would prevent developers from using
> add_memory() without the lock, say.
> 
> If the lock is always going to be required for add_memory(), make it
> hard (or event impossible) to use the latter without it.
> 

If there are no objections, I'll go into that direction. But I'll wait
for more comments regarding the general concept first.

Thanks!

-- 

Thanks,

David / dhildenb

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