Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754620Ab3IZXH7 (ORCPT ); Thu, 26 Sep 2013 19:07:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:24929 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754485Ab3IZXH5 (ORCPT ); Thu, 26 Sep 2013 19:07:57 -0400 Date: Fri, 27 Sep 2013 01:05:40 +0200 From: Veaceslav Falico To: Bjorn Helgaas Cc: Neil Horman , "linux-kernel@vger.kernel.org" , "linux-pci@vger.kernel.org" , Greg Kroah-Hartman , Russell King Subject: Re: [PATCH 1/3] msi: add forgotten pci_dev_put(pdev) to populate_msi_sysfs() Message-ID: <20130926230540.GB17654@redhat.com> References: <1379382464-7920-1-git-send-email-vfalico@redhat.com> <1379382464-7920-2-git-send-email-vfalico@redhat.com> <20130925232300.GA7368@neilslaptop.think-freely.org> <20130926122552.GB4783@redhat.com> <20130926140751.GC4783@redhat.com> <20130926221613.GA24611@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20130926221613.GA24611@google.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6022 Lines: 141 On Thu, Sep 26, 2013 at 04:16:13PM -0600, Bjorn Helgaas wrote: >[+cc Russell] > >On Thu, Sep 26, 2013 at 04:07:51PM +0200, Veaceslav Falico wrote: >> On Thu, Sep 26, 2013 at 02:25:52PM +0200, Veaceslav Falico wrote: >> >On Wed, Sep 25, 2013 at 05:35:54PM -0600, Bjorn Helgaas wrote: >> >>On Wed, Sep 25, 2013 at 5:23 PM, Neil Horman wrote: >> >>>On Wed, Sep 25, 2013 at 03:08:05PM -0600, Bjorn Helgaas wrote: >> >>>>On Mon, Sep 16, 2013 at 7:47 PM, Veaceslav Falico wrote: >... >> >As an update - I've found an interesting case why exactly that >> >kobject_del() might be needed: >> > >> >in kobject_del() it removes instantly the link to kset - via >> >kobj_kset_leave(), so that our kset remains without links and, thus, might >> >be instantly removed. >> > >> >So, with kobject_del(), our kset (msi_irqs sysfs dir) remains instantly >> >without any links (i.e. other kobjects) and, when we call kset_unregister() >> >- it exits instantly (if it's not being hold somewhere elsewhere...). >> > >> >Without it, kset_unregister() will wait till all the kobjects will be gone. > >I don't see any waiting in kset_unregister(); all it does is a >kobject_put(). Sorry, didn't word it correctly - my thought was that kset_unregister() (which is, basicly, kobject_put()) will NOT unregister it instantly without kobject_del() in place, because the 'child' kobjects are still tied to this kset. > >> >Now, the fun part starts - if we quickly call pci_disable_msi() and, >> >afterwards, pci_enable_msi() - we might fail because the msi_irqs kset is >> >still there, waiting to unregister, and the sysfs dir is still active. >> > >> >It's used, for example, in tg3_open/tg3_close, which are ndo_open/close, >> >and are called on enslave/deslave in bonding. >> > >> >What I get: >> >[ 60.458319] WARNING: CPU: 0 PID: 5552 at fs/sysfs/dir.c:526 sysfs_add_one+0xbb/0xe0() >> >[ 60.458350] sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:1c.5/0000:3f:00.0/msi_irqs' >> > >> >I'll take a deeper look at the issue, though any feedback/advise is >> >welcome. And I'll hold on with the patchset that removes pci_dev_get/put >> >and kobject_del. >> >> Ok, this is only reproducible with CONFIG_DEBUG_KOBJECT_RELEASE, with or >> without removing kobject_del() (though it's harder to reproduce). I could >> not trigger it without CONFIG_DEBUG_KOBJECT_RELEASE, even with constantly >> poking /sys/class/net/tg3_device/device/msi_irqs/* . Though it's, >> obviously, possible, with and without cleanup and my previous bugfix. >> >> I'll then send now the cleanup, however this theoretical issue was, is and >> won't be fixed by it :-/. And I don't know how can we possible fix it >> without something like kobject_put_wait(). > >I still think we should remove the kobject_del(). We don't want to >make a race harder to hit; we want to remove it completely. What we >really want is to make races *easier* to hit so we can find them, >which seems to be the point of KOBJECT_RELEASE :) Agree, that's what I've done in the v2 patchset :). > >That said, I think I see why you see the warning in this case. >You're calling pci_enable_msi(), pci_disable_msi(), pci_enable_msi() >as in this call chain: > > pci_enable_msi > msi_capability_init > populate_msi_sysfs > dev->msi_kset = kset_create_and_add("msi_irqs", ...) > list_for_each_entry(entry, &dev->msi_list, ...) > kobj->kset = dev->msi_kset > kobject_init_and_add(kobj, &msi_irq_ktype, NULL, ...) > kobject_add_internal > kobject_get(&kobj->kset->kobj) # dev->msi_kset > > pci_disable_msi > free_msi_irqs > list_for_each_entry_safe(entry, ..., &dev->msi_list, ...) > kobject_put(&entry->kobj) > kobject_release > ... ... > kobject_cleanup > kobject_del > kobj_kset_leave > kset_put(kobj->kset) # dev->msi_kset > kobject_put # happens AFTER pci_enable_msi() below > t->release > list_del(&entry->list) > kset_unregister(dev->msi_kset) > kobject_put > kobject_release > ... ... > kobject_cleanup # happens AFTER pci_enable_msi() below > dev->msi_kset = NULL > > pci_enable_msi > msi_capability_init > populate_msi_sysfs > dev->msi_kset = kset_create_and_add("msi_irqs", ...) > "sysfs: cannot create duplicate filename .../msi_irqs" > >When we kset_create_and_add("msi_irqs") the second time, the delayed >kobject_cleanups() for the msi_desc entries and the msi_kset have >not yet occurred, so the msi_desc entries still hold references to >the msi_kset, etc. Exactly! > >I'm not sure if this is a design problem in the way PCI manages >msi_kset and msi_desc entries, or if there's something wrong in >the way KOBJECT_RELEASE is implemented. I could imagine changing >it so the bulk of kobject_cleanup(), including the sysfs cleanup, >is executed immediately when the last reference is dropped, but >the kobj_type->release() function itself is delayed. Yep, could be one of the possibilities - and it actually resembles what kobject_del() does :). But anyway, I agree that we shouldn't leave kobject_del(), cause it only hides the real problem, and not completely (if there are some other users who kobject_get(kset/kobject) - we're in trouble). > >Calling kobject_del() explicitly sort of side-steps this problem >by doing the sysfs cleanup before the last put. But it is quite >subtle, and it feels error-prone to rely on that. So, in my v2 patchset, I've removed the kobject_del(). It's really hard to trigger the bug without KOBJECT_DEBUG_RELEASE, still. > >Bjorn -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/