Re: [PATCH 1/3] msi: add forgotten pci_dev_put(pdev) topopulate_msi_sysfs()

From: Veaceslav Falico
Date: Thu Sep 26 2013 - 19:08:04 EST

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 <nhorman@xxxxxxxxxxxxx> 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 <vfalico@xxxxxxxxxx> 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
... <delayed> ...
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
... <delayed> ...
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
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