Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH RFC 1/2] drivers/base: export
 lock_device_hotplug/unlock_device_hotplug
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        devel@linuxdriverproject.org, linux-s390@vger.kernel.org,
        xen-devel@lists.xenproject.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Pavel Tatashin <pasha.tatashin@oracle.com>, osalvador@suse.de,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Len Brown <lenb@kernel.org>,
        David Rientjes <rientjes@google.com>
References: <20180817075901.4608-1-david@redhat.com>
 <20180817075901.4608-2-david@redhat.com> <20180817084146.GB14725@kroah.com>
 <5a5d73e9-e4aa-ffed-a2e3-8aef64e61923@redhat.com>
 <CAJZ5v0gkYV8o2Eq+EcGT=OP1tQGPGVVe3n9VGD6z7KAVVqhv9w@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Openpgp: preference=signencrypt
Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI
 BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG
 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj
 xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3
 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob
 s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM
 m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez
 MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK
 z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh
 dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E
 UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq
 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga
 uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid
 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X
 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+
 xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC
 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb
 hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC
 u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS
 gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg
 rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl
 BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl
 KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ
 NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo
 YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5
 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy
 qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl
 C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV
 W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV
 TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx
 +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl
 SE+xAvmumFBY
Organization: Red Hat GmbH
Message-ID: <42df9062-f647-3ad6-5a07-be2b99531119@redhat.com>
Date:   Fri, 17 Aug 2018 11:41:24 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <CAJZ5v0gkYV8o2Eq+EcGT=OP1tQGPGVVe3n9VGD6z7KAVVqhv9w@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Thanks!

-- 

Thanks,

David / dhildenb