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Message-ID: <484DEF46.7080106@jp.fujitsu.com>
Date: Tue, 10 Jun 2008 12:04:38 +0900
From: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
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To: Alex Chiang <achiang@hp.com>,
       Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>,
       jbarnes@virtuousgeek.org,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Matthew Wilcox <matthew@wil.cx>,
       Andrew Morton <akpm@linux-foundation.org>, kristen.c.accardi@intel.com,
       greg@kroah.com, lenb@kernel.org, pbadari@us.ibm.com,
       linux-pci@vger.kernel.org, pcihpd-discuss@lists.sourceforge.net,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/4, v14] PCI, ACPI: Physical PCI slot objects
References: <20080604200829.GC379@ldl.fc.hp.com> <484CE515.4030205@jp.fujitsu.com> <20080609221108.GA16588@ldl.fc.hp.com>
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Alex Chiang wrote:
> * Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>:
>> I tried your patches and I have a comment (question) about the behavior.
>>
>> To emulate the (broken?) platform that assigns the same name to multiple
>> slots, I made a debug patch for pciehp driver to assign same name ('1000')
>> to all slots (my environment has two PCIe slots). With this patch, I
>> noticed that the behavior or pci_hp_register() (or pci_create_slot())
>> varies depending on whether pci_slot driver is loaded or not. See below.
>>
>> (a) With pci_slot driver loaded
>>    I got the following error message when I loaded pciehp driver.
>>
>>        pciehp: pci_hp_register failed with error -17
>>        pciehp: Failed to register slot because of name collision. Try
>>        'pciehp_slot_with_bus' module option.
>>        pciehp: pciehp: slot initialization failed
>>      (b) Without pci_slot driver loaded
>>    I got the kernel stack dump and error messages when I loaded pciehp
>>    driver.
>>
>> 	kobject_add_internal failed for 1000 with -EEXIST, don't try to
>> 	register things with the same name in the same directory.
>>
>> 	Call Trace:
>> 	 [<a000000100015180>] show_stack+0x40/0xa0
>> 	                                sp=e0000040a086fb80 bsp=e0000040a0861158
>> 	 [<a000000100015210>] dump_stack+0x30/0x60
>>                                sp=e0000040a086fd50 bsp=e0000040a0861140
>> 	 [<a0000001003b3910>] kobject_add_internal+0x330/0x400
>> 	                                sp=e0000040a086fd50 bsp=e0000040a0861100
>> 	 [<a0000001003b3bd0>] kobject_add_varg+0x90/0xc0
>> 	                                sp=e0000040a086fd50 bsp=e0000040a08610c8
>> 	 [<a0000001003b3c90>] kobject_init_and_add+0x90/0xc0
>> 	                                sp=e0000040a086fd50 bsp=e0000040a0861068
>> 	 [<a0000001003d69b0>] pci_create_slot+0x150/0x260
>> 	                                sp=e0000040a086fd80 bsp=e0000040a0861030
>> 	 [<a000000200b71870>] pci_hp_register+0x130/0x880 [pci_hotplug]
>> 	                                sp=e0000040a086fd80 bsp=e0000040a0860ff0
>> 	 [<a000000200ec1a60>] pciehp_probe+0x720/0xca0 [pciehp]
>>
>> 	(snip...)
>>
>> 	Unable to register kobject 1000
>> 	pciehp: pci_hp_register failed with error -17
>> 	pciehp: Failed to register slot because of name collision. Try  
>> 	'pciehp_slot_with_bus' module option.
>> 	pciehp: pciehp: slot initialization failed
>>
>> Could you tell me why that difference happen? And my expectation is
>> the result should be (a) above regardless of whether pci_slot driver
>> is loaded or not.
> 
> The difference was because in (a), pci_slot created the slots and
> when pciehp tried to register them, the pci slot infrastructure
> simply refcounted them and returned, but did not try to
> re-register the kobjects with the kobj core.
> 
> In (b), the pci hotplug core allowed you to create multiple slots
> with the same name, and called pci_create_slot() multiple times.
> This time, since you were trying to create new slot objects, we
> did not refcount them; we actually did a kzalloc *and* tried to
> register them with the kobject core, which is why we got that
> stack trace.
> 
> I read your patch (a86161b3134465f) in closer detail and decided
> that it can work without problems with my patch series. 
> 
> 	- Your patch will keep track of hotplug slots and
> 	  disallow multiple hotplug slots with the same name
> 
> 	- the PCI slot infrastructure will still allow multiple
> 	  callers of pci_create_slot() to succeed by refcounting
> 	  identical slots
> 
> This is the correct behavior to allow pciehp and pci_slot to
> coexist because pci_slot is not trying to register a hotplug
> handler or do any other hotplug operations.
> 

Thank you for explanation. I understood.

But I have one concern about the behavior when pci slot driver is not
loaded. My patch (a86161b3134465f) prevents the kernel trace dump
because of duplicate kobject add that would happen in the following two
cases, though (b) is not described in the header of the patch (sorry).

  (a) multiple driver attempt to handle the same slot.

  (b) one or more driver attempt to register multiple slots with the
      same name (This can happen if broken platform assigns the same
      slot number to multiple hotplug slots, for example).

With your patch, duplicate kobject add in case (b) is not prevented.
That is my concern.

I made a below patch to prevent (b), please take a look. And could you
please consider merging it to "[PATCH 2/3] Introduce pci_slot" in your
latest series.

Thanks,
Kenji Kaneshige


---
 drivers/pci/hotplug/pci_hotplug_core.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

Index: 20080610/drivers/pci/hotplug/pci_hotplug_core.c
===================================================================
--- 20080610.orig/drivers/pci/hotplug/pci_hotplug_core.c
+++ 20080610/drivers/pci/hotplug/pci_hotplug_core.c
@@ -555,6 +555,7 @@ int pci_hp_register(struct hotplug_slot 
 {
 	int result;
 	struct pci_slot *pci_slot;
+	struct hotplug_slot *tmp;
 
 	if (slot == NULL)
 		return -ENODEV;
@@ -567,6 +568,21 @@ int pci_hp_register(struct hotplug_slot 
 	}
 
 	/*
+	 * Prevent registering multiple hotplug slots with the same name.
+	 */
+	spin_lock(&pci_hotplug_slot_list_lock);
+	list_for_each_entry(tmp, &pci_hotplug_slot_list, slot_list) {
+		pci_slot = tmp->pci_slot;
+		if (pci_slot->bus == bus && pci_slot->number == slot_nr)
+			continue;
+		if (!strcmp(tmp->name, slot->name)) {
+			spin_unlock(&pci_hotplug_slot_list_lock);
+			return -EEXIST;
+		}
+	}
+	spin_unlock(&pci_hotplug_slot_list_lock);
+
+	/*
 	 * No problems if we call this interface from both ACPI_PCI_SLOT
 	 * driver and call it here again. If we've already created the
 	 * pci_slot, the interface will simply bump the refcount.


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