Re: [PATCH 01/40] PCI: fix default vga ref_count

[Yinghai Lu <yinghai@kernel.org>]

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On Fri, Sep 21, 2012 at 1:52 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
> On Wed, Sep 19, 2012 at 12:54 PM, Yinghai Lu <yinghai@kernel.org> wrote:
>> --- a/drivers/gpu/vga/vgaarb.c
>> +++ b/drivers/gpu/vga/vgaarb.c
>> @@ -141,6 +141,12 @@ EXPORT_SYMBOL_GPL(vga_default_device);
>>
>>  void vga_set_default_device(struct pci_dev *pdev)
>>  {
>> +       if (vga_default)
>> +               pci_dev_put(vga_default);
>> +
>> +       if (pdev)
>> +               pdev = pci_dev_get(pdev);
>> +
>>         vga_default = pdev;
>
> I think this is equivalent to:
>
>     pci_dev_put(vga_default);
>     vga_default = pci_dev_get(pdev);

ah, i missed that, pci_dev_put and pci_dev_get check that inside.

>
> I haven't seen an answer to Matthew's question about whether we want
> to check for "vga_default == pdev".

yes, that could avoid the extra put/get pair.

Please check updated version.

-Yinghai

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Subject: [PATCH] PCI: fix default vga ref_count

when __ARCH_HAS_VGA_DEFAULT_DEVICE is not defined, aka EFIFB is not used,
for static path, vga_default setting is through vga_arbiter_add_pci_device.
and later x86 pci_fixup_video, will skip setting again.
- subsys_initcall(vga_arb_device_init) come first to call vga_arbiter_add_pci_device. It will call pci_get_dev to hold one reference.

for hotplug add path, even vga_arbiter_add_pci_device is called via notifier,
but it will check VGA_RSRC_LEGACY_MASK that is not set for hotplug path.
So x86 pci_fixup_video will take over to call vga_set_default_device().
It will not hold one refrence.

Later for hotplug remove path, vga_arbiter_del_pci_device that does not check
VGA_RSRC_LEGACY_MASK will call put_device and it will cause ref_count to
decrease extra. that will have that pci device get deleted early wrongly.

Need to make get/put balance for both cases.

-v2: According to Bjorn, update vga_set_default_device instead...
-v3: Remove extra checking about null that pointed out by Bjorn.
     Add extra check for same value, to avoid extra put/get pair, suggested by
	Matthew.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: x86@kernel.org
Cc: Dave Airlie <airlied@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Julia Lawall <julia@diku.dk>
Cc: Matthew Garrett <mjg@redhat.com>

---
 drivers/gpu/vga/vgaarb.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

Index: linux-2.6/drivers/gpu/vga/vgaarb.c
===================================================================
--- linux-2.6.orig/drivers/gpu/vga/vgaarb.c
+++ linux-2.6/drivers/gpu/vga/vgaarb.c
@@ -141,7 +141,11 @@ EXPORT_SYMBOL_GPL(vga_default_device);
 
 void vga_set_default_device(struct pci_dev *pdev)
 {
-	vga_default = pdev;
+	if (vga_default == pdev)
+		return;
+
+	pci_dev_put(vga_default);
+	vga_default = pci_dev_get(pdev);
 }
 #endif
 
@@ -577,7 +581,7 @@ static bool vga_arbiter_add_pci_device(s
 #ifndef __ARCH_HAS_VGA_DEFAULT_DEVICE
 	if (vga_default == NULL &&
 	    ((vgadev->owns & VGA_RSRC_LEGACY_MASK) == VGA_RSRC_LEGACY_MASK))
-		vga_default = pci_dev_get(pdev);
+		vga_set_default_device(pdev);
 #endif
 
 	vga_arbiter_check_bridge_sharing(vgadev);
@@ -613,10 +617,8 @@ static bool vga_arbiter_del_pci_device(s
 	}
 
 #ifndef __ARCH_HAS_VGA_DEFAULT_DEVICE
-	if (vga_default == pdev) {
-		pci_dev_put(vga_default);
-		vga_default = NULL;
-	}
+	if (vga_default == pdev)
+		vga_set_default_device(NULL);
 #endif
 
 	if (vgadev->decodes & (VGA_RSRC_LEGACY_IO | VGA_RSRC_LEGACY_MEM))