From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f197.google.com (mail-qk0-f197.google.com [209.85.220.197]) by kanga.kvack.org (Postfix) with ESMTP id 374886B080F for ; Fri, 17 Aug 2018 07:56:41 -0400 (EDT) Received: by mail-qk0-f197.google.com with SMTP id c27-v6so7372025qkj.3 for ; Fri, 17 Aug 2018 04:56:41 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id k1-v6si1752321qvf.138.2018.08.17.04.56.40 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 17 Aug 2018 04:56:40 -0700 (PDT) Subject: Re: [PATCH RFC 1/2] drivers/base: export lock_device_hotplug/unlock_device_hotplug References: <20180817075901.4608-1-david@redhat.com> <20180817075901.4608-2-david@redhat.com> <20180817084146.GB14725@kroah.com> <5a5d73e9-e4aa-ffed-a2e3-8aef64e61923@redhat.com> <42df9062-f647-3ad6-5a07-be2b99531119@redhat.com> <20180817100604.GA18164@kroah.com> <4ac624be-d2d6-5975-821f-b20a475781dc@redhat.com> <20180817112850.GB3565@osiris> From: David Hildenbrand Message-ID: Date: Fri, 17 Aug 2018 13:56:35 +0200 MIME-Version: 1.0 In-Reply-To: <20180817112850.GB3565@osiris> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Heiko Carstens Cc: Greg Kroah-Hartman , "Rafael J. Wysocki" , Michal Hocko , Benjamin Herrenschmidt , Linux Memory Management List , Paul Mackerras , linux-s390@vger.kernel.org, sthemmin@microsoft.com, Pavel Tatashin , ACPI Devel Maling List , David Rientjes , xen-devel@lists.xenproject.org, Len Brown , haiyangz@microsoft.com, Dan Williams , Andrew Morton , Vlastimil Babka , osalvador@suse.de, "Rafael J. Wysocki" , Linux Kernel Mailing List , Martin Schwidefsky , devel@linuxdriverproject.org, Vitaly Kuznetsov , linuxppc-dev On 17.08.2018 13:28, Heiko Carstens wrote: > On Fri, Aug 17, 2018 at 01:04:58PM +0200, David Hildenbrand wrote: >>>> 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 :) > > I hope you figured out what needs to be locked why. Your patch description > seems to be "only" about locking order ;) Well I hope so, too ... but there is a reason for the RFC mark ;) There is definitely a lot of magic in the current code. And that's why it is also not that obvious that locking is wrong. To avoid/fix the locking order problem was the motivation for the original patch that dropped mem_hotplug_lock on one path. So I focused on that in my description. > > I tried to figure out and document that partially with 55adc1d05dca ("mm: > add private lock to serialize memory hotplug operations"), and that wasn't > easy to figure out. I was especially concerned about sprinkling Haven't seen that so far as that was reworked by 3f906ba23689 ("mm/memory-hotplug: switch locking to a percpu rwsem"). Thanks for the pointer. There is a long history to all this. > lock/unlock_device_hotplug() calls, which has the potential to make it the > next BKL thing. Well, the thing with memory hotplug and device_hotplug_lock is that a) ACPI already holds it while adding/removing memory via add_memory() b) we hold it during online/offline of memory (via sysfs calls to device_online()/device_offline()) So it is already pretty much involved in all memory hotplug/unplug activities on x86 (except paravirt). And as far as I understand, there are good reasons to hold the lock in core.c and ACPI. (as mentioned by Rafael) The exceptions are add_memory() called on s390x, hyper-v, xen and ppc (including manual probing). And device_online()/device_offline() called from the kernel. Holding device_hotplug_lock when adding/removing memory from the system doesn't sound too wrong (especially as devices are created/removed). At least that way (documenting and following the rules in the patch description) we might at least get locking right. I am very open to other suggestions (but as noted by Greg, many maintainers might be busy by know). E.g. When adding the memory block devices, we know that there won't be a driver to attach to (as there are no drivers for the "memory" subsystem) - the bus_probe_device() function that takes the device_lock() could pretty much be avoided for that case. But burying such special cases down in core driver code definitely won't make locking related to memory hotplug easier. Thanks for having a look! -- Thanks, David / dhildenb