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Subject: Re: [PATCH RFC 1/2] drivers/base: export
 lock_device_hotplug/unlock_device_hotplug
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Paul Mackerras <paulus@samba.org>, linux-s390@vger.kernel.org,
        sthemmin@microsoft.com, Pavel Tatashin <pasha.tatashin@oracle.com>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        David Rientjes <rientjes@google.com>,
        xen-devel@lists.xenproject.org, Len Brown <lenb@kernel.org>,
        haiyangz@microsoft.com, Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, osalvador@suse.de,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        devel@linuxdriverproject.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <4ac624be-d2d6-5975-821f-b20a475781dc@redhat.com>
Date:   Fri, 17 Aug 2018 13:04:58 +0200
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On 17.08.2018 12:06, Greg Kroah-Hartman wrote:
> On Fri, Aug 17, 2018 at 11:41:24AM +0200, David Hildenbrand wrote:
>> On 17.08.2018 11:03, Rafael J. Wysocki wrote:
>>> On Fri, Aug 17, 2018 at 10:56 AM David Hildenbrand <david@redhat.com> 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@redhat.com>
>>>>>>
>>>>>> 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@redhat.com>
>>>>>> [modify patch description]
>>>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>>>> ---
>>>>>>  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.
> 
> It is the middle of the merge window, and maintainers are really busy
> right now.  I doubt you will get many review comments just yet...
> 

This has been broken since 2015, so I guess it can wait a bit :)

-- 

Thanks,

David / dhildenb