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[209.132.183.28]) by mx.google.com with ESMTPS id h17si27334834qkg.231.2019.07.26.00.21.02 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 26 Jul 2019 00:21:02 -0700 (PDT) Received-SPF: pass (google.com: domain of david@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of david@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 8121DA70E; Fri, 26 Jul 2019 07:21:01 +0000 (UTC) Received: from [10.36.116.244] (ovpn-116-244.ams2.redhat.com [10.36.116.244]) by smtp.corp.redhat.com (Postfix) with ESMTP id B6A3D5DE80; Fri, 26 Jul 2019 07:20:59 +0000 (UTC) Subject: Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan_init() To: "Rafael J. Wysocki" Cc: Michal Hocko , Linux Kernel Mailing List , Linux Memory Management List , ACPI Devel Maling List , "Rafael J. Wysocki" , Andrew Morton , Oscar Salvador References: <20190724143017.12841-1-david@redhat.com> <20190725125636.GA3582@dhcp22.suse.cz> <6dc566c2-faf6-565d-4ef1-2ac3a366bc76@redhat.com> <20190725135747.GB3582@dhcp22.suse.cz> <447b74ca-f7c7-0835-fd50-a9f7191fe47c@redhat.com> <20190725191943.GA6142@dhcp22.suse.cz> From: David Hildenbrand 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: <8e891e0c-9024-b5ad-0f44-bccd4e87c60e@redhat.com> Date: Fri, 26 Jul 2019 09:20:58 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Fri, 26 Jul 2019 07:21:01 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 25.07.19 23:23, Rafael J. Wysocki wrote: > On Thu, Jul 25, 2019 at 10:49 PM David Hildenbrand wrote: >> >> On 25.07.19 21:19, Michal Hocko wrote: >>> On Thu 25-07-19 16:35:07, David Hildenbrand wrote: >>>> On 25.07.19 15:57, Michal Hocko wrote: >>>>> On Thu 25-07-19 15:05:02, David Hildenbrand wrote: >>>>>> On 25.07.19 14:56, Michal Hocko wrote: >>>>>>> On Wed 24-07-19 16:30:17, David Hildenbrand wrote: >>>>>>>> We end up calling __add_memory() without the device hotplug lock held. >>>>>>>> (I used a local patch to assert in __add_memory() that the >>>>>>>> device_hotplug_lock is held - I might upstream that as well soon) >>>>>>>> >>>>>>>> [ 26.771684] create_memory_block_devices+0xa4/0x140 >>>>>>>> [ 26.772952] add_memory_resource+0xde/0x200 >>>>>>>> [ 26.773987] __add_memory+0x6e/0xa0 >>>>>>>> [ 26.775161] acpi_memory_device_add+0x149/0x2b0 >>>>>>>> [ 26.776263] acpi_bus_attach+0xf1/0x1f0 >>>>>>>> [ 26.777247] acpi_bus_attach+0x66/0x1f0 >>>>>>>> [ 26.778268] acpi_bus_attach+0x66/0x1f0 >>>>>>>> [ 26.779073] acpi_bus_attach+0x66/0x1f0 >>>>>>>> [ 26.780143] acpi_bus_scan+0x3e/0x90 >>>>>>>> [ 26.780844] acpi_scan_init+0x109/0x257 >>>>>>>> [ 26.781638] acpi_init+0x2ab/0x30d >>>>>>>> [ 26.782248] do_one_initcall+0x58/0x2cf >>>>>>>> [ 26.783181] kernel_init_freeable+0x1bd/0x247 >>>>>>>> [ 26.784345] kernel_init+0x5/0xf1 >>>>>>>> [ 26.785314] ret_from_fork+0x3a/0x50 >>>>>>>> >>>>>>>> So perform the locking just like in acpi_device_hotplug(). >>>>>>> >>>>>>> While playing with the device_hotplug_lock, can we actually document >>>>>>> what it is protecting please? I have a bad feeling that we are adding >>>>>>> this lock just because some other code path does rather than with a good >>>>>>> idea why it is needed. This patch just confirms that. What exactly does >>>>>>> the lock protect from here in an early boot stage. >>>>>> >>>>>> We have plenty of documentation already >>>>>> >>>>>> mm/memory_hotplug.c >>>>>> >>>>>> git grep -C5 device_hotplug mm/memory_hotplug.c >>>>>> >>>>>> Also see >>>>>> >>>>>> Documentation/core-api/memory-hotplug.rst >>>>> >>>>> OK, fair enough. I was more pointing to a documentation right there >>>>> where the lock is declared because that is the place where people >>>>> usually check for documentation. The core-api documentation looks quite >>>>> nice. And based on that doc it seems that this patch is actually not >>>>> needed because neither the online/offline or cpu hotplug should be >>>>> possible that early unless I am missing something. >>>> >>>> I really prefer to stick to locking rules as outlined on the >>>> interfaces if it doesn't hurt. Why it is not needed is not clear. >>>> >>>>> >>>>>> Regarding the early stage: primarily lockdep as I mentioned. >>>>> >>>>> Could you add a lockdep splat that would be fixed by this patch to the >>>>> changelog for reference? >>>>> >>>> >>>> I have one where I enforce what's documented (but that's of course not >>>> upstream and therefore not "real" yet) >>> >>> Then I suppose to not add locking for something that is not a problem. >>> Really, think about it. People will look at this code and follow the >>> lead without really knowing why the locking is needed. >>> device_hotplug_lock has its purpose and if the code in question doesn't >>> need synchronization for the documented scenarios then the locking >>> simply shouldn't be there. Adding the lock just because of a >>> non-existing, and IMHO dubious, lockdep splats is just wrong. >>> >>> We need to rationalize the locking here, not to add more hacks. >> >> No, sorry. The real hack is calling a function that is *documented* to >> be called under lock without it. That is an optimization for a special >> case. That is the black magic in the code. >> >> The only alternative I see to this patch is adding a comment like >> >> /* >> * We end up calling __add_memory() without the device_hotplug_lock >> * held. This is fine as we cannot race with other hotplug activities >> * and userspace trying to online memory blocks. >> */ >> >> Personally, I don't think that's any better than just grabbing the lock >> as we are told to. (honestly, I don't see how optimizing away the lock >> here is of *any* help to optimize our overall memory hotplug locking) >> >> @Rafael, what's your take? lock or comment? > > Well, I have ACKed your patch already. :-) It's never to late to un-ACK if you changed your mind :) > > That said, adding a comment stating that the lock is acquired mostly > for consistency wouldn't hurt. > I can certainly do that. Thanks! -- Thanks, David / dhildenb