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[209.132.183.28]) by mx.google.com with ESMTPS id 10si32608951qtv.339.2019.07.26.01.22.39 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 26 Jul 2019 01:22:39 -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-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D272C4E908; Fri, 26 Jul 2019 08:22:38 +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 1BD5A6092D; Fri, 26 Jul 2019 08:22:36 +0000 (UTC) Subject: Re: [PATCH RFC] mm/memory_hotplug: Don't take the cpu_hotplug_lock To: Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Oscar Salvador , Pavel Tatashin , Dan Williams , Thomas Gleixner References: <20190725092206.23712-1-david@redhat.com> <20190726081919.GI6142@dhcp22.suse.cz> From: David Hildenbrand Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190726081919.GI6142@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Fri, 26 Jul 2019 08:22:38 +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 26.07.19 10:19, Michal Hocko wrote: > On Thu 25-07-19 11:22:06, David Hildenbrand wrote: >> Commit 9852a7212324 ("mm: drop hotplug lock from lru_add_drain_all()") >> states that lru_add_drain_all() "Doesn't need any cpu hotplug locking >> because we do rely on per-cpu kworkers being shut down before our >> page_alloc_cpu_dead callback is executed on the offlined cpu." >> >> And also "Calling this function with cpu hotplug locks held can actually >> lead to obscure indirect dependencies via WQ context.". >> >> Since commit 3f906ba23689 ("mm/memory-hotplug: switch locking to a percpu >> rwsem") we do a cpus_read_lock() in mem_hotplug_begin(). >> >> I don't see how that lock is still helpful, we already hold the >> device_hotplug_lock to protect try_offline_node(), which is AFAIK one >> problematic part that can race with CPU hotplug. If it is still >> necessary, we should document why. > > I have forgot all the juicy details. Maybe Thomas remembers. The > previous recursive home grown locking was just terrible. I do not see > stop_machine being used in the memory hotplug anymore. > > I do support this kind of removal because binding CPU and MEM hotplug > locks is fragile and wrong. But this patch really needs more explanation > on why this is safe. In other words what does cpu_read_lock protects > from in mem hotplug paths. And that is the purpose of marking this RFC, because I am not aware of any :) Hopefully Thomas can clarify if we are missing something important (undocumented) here - if so I'll document it. -- Thanks, David / dhildenb