From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f69.google.com (mail-oi0-f69.google.com [209.85.218.69]) by kanga.kvack.org (Postfix) with ESMTP id C691A6B0770 for ; Fri, 17 Aug 2018 05:03:22 -0400 (EDT) Received: by mail-oi0-f69.google.com with SMTP id w185-v6so6595298oig.19 for ; Fri, 17 Aug 2018 02:03:22 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id 16-v6sor861278oij.170.2018.08.17.02.03.21 for (Google Transport Security); Fri, 17 Aug 2018 02:03:21 -0700 (PDT) MIME-Version: 1.0 References: <20180817075901.4608-1-david@redhat.com> <20180817075901.4608-2-david@redhat.com> <20180817084146.GB14725@kroah.com> <5a5d73e9-e4aa-ffed-a2e3-8aef64e61923@redhat.com> In-Reply-To: <5a5d73e9-e4aa-ffed-a2e3-8aef64e61923@redhat.com> From: "Rafael J. Wysocki" Date: Fri, 17 Aug 2018 11:03:09 +0200 Message-ID: Subject: Re: [PATCH RFC 1/2] drivers/base: export lock_device_hotplug/unlock_device_hotplug Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: Greg Kroah-Hartman , Linux Kernel Mailing List , Linux Memory Management List , linuxppc-dev , ACPI Devel Maling List , devel@linuxdriverproject.org, linux-s390@vger.kernel.org, xen-devel@lists.xenproject.org, Andrew Morton , Michal Hocko , Vlastimil Babka , Dan Williams , Pavel Tatashin , osalvador@suse.de, Vitaly Kuznetsov , Martin Schwidefsky , Heiko Carstens , kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com, Benjamin Herrenschmidt , Paul Mackerras , "Rafael J. Wysocki" , Len Brown , David Rientjes On Fri, Aug 17, 2018 at 10:56 AM David Hildenbrand 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 > >> > >> 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 > >> [modify patch description] > >> Signed-off-by: David Hildenbrand > >> --- > >> 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.