[PATCH v2] cxl: Fix unbalanced pci_dev_get in cxl_probe

[PATCH v2] cxl: Fix unbalanced pci_dev_get in cxl_probe

[Daniel Axtens]

Currently the first thing we do in cxl_probe is to grab a reference
on the pci device. Later on, we call device_register on our adapter.
In our remove path, we call device_unregister, but we never call
pci_dev_put. We therefore leak the device every time we do a
reflash.

device_register/unregister is sufficient to hold the reference.
Therefore, drop the call to pci_dev_get.

Here's why this is safe.
The proposed cxl_probe(pdev) calls cxl_adapter_init:
    a) init calls cxl_adapter_alloc, which creates a struct cxl,
       conventionally called adapter. This struct contains a
       device entry, adapter->dev.

    b) init calls cxl_configure_adapter, where we set
       adapter->dev.parent = &dev->dev (here dev is the pci dev)

So at this point, the cxl adapter's device's parent is the PCI
device that I want to be refcounted properly.

    c) init calls cxl_register_adapter
       *) cxl_register_adapter calls device_register(&adapter->dev)

So now we're in device_register, where dev is the adapter device, and
we want to know if the PCI device is safe after we return.

device_register(&adapter->dev) calls device_initialize() and then
device_add().

device_add() does a get_device(). device_add() also explicitly grabs
the device's parent, and calls get_device() on it:

         parent = get_device(dev->parent);

So therefore, device_register() takes a lock on the parent PCI dev,
which is what pci_dev_get() was guarding. pci_dev_get() can therefore
be safely removed.

Fixes: f204e0b8cedd ("cxl: Driver code for powernv PCIe based cards for userspace access")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Axtens <dja@axtens.net>

---

This is the cxl bug that caused me to catch this a few weeks back:
e642d11bdbfe ("powerpc/eeh: Probe after unbalanced kref check")

I put an printk in the unbalanced kref path and confirmed that it
was printed with the pci_dev_get in and went away with the
pci_dev_get out.
---
 drivers/misc/cxl/pci.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/misc/cxl/pci.c b/drivers/misc/cxl/pci.c
index 02c85160bfe9..a5e977192b61 100644
--- a/drivers/misc/cxl/pci.c
+++ b/drivers/misc/cxl/pci.c
@@ -1249,8 +1249,6 @@ static int cxl_probe(struct pci_dev *dev, const struct pci_device_id *id)
 	int slice;
 	int rc;
 
-	pci_dev_get(dev);
-
 	if (cxl_verbose)
 		dump_cxl_config_space(dev);
 
-- 
2.5.0

Re: [PATCH v2] cxl: Fix unbalanced pci_dev_get in cxl_probe

[Ian Munsie]

Acked-by: Ian Munsie <imunsie@au1.ibm.com>

Re: [v2] cxl: Fix unbalanced pci_dev_get in cxl_probe

[Michael Ellerman]

On Tue, 2015-15-09 at 05:04:07 UTC, Daniel Axtens wrote:
> Currently the first thing we do in cxl_probe is to grab a reference
> on the pci device. Later on, we call device_register on our adapter.
> In our remove path, we call device_unregister, but we never call
> pci_dev_put. We therefore leak the device every time we do a
> reflash.
> 
> device_register/unregister is sufficient to hold the reference.
> Therefore, drop the call to pci_dev_get.
> 
> Here's why this is safe.
> The proposed cxl_probe(pdev) calls cxl_adapter_init:
>     a) init calls cxl_adapter_alloc, which creates a struct cxl,
>        conventionally called adapter. This struct contains a
>        device entry, adapter->dev.
> 
>     b) init calls cxl_configure_adapter, where we set
>        adapter->dev.parent = &dev->dev (here dev is the pci dev)
> 
> So at this point, the cxl adapter's device's parent is the PCI
> device that I want to be refcounted properly.
> 
>     c) init calls cxl_register_adapter
>        *) cxl_register_adapter calls device_register(&adapter->dev)
> 
> So now we're in device_register, where dev is the adapter device, and
> we want to know if the PCI device is safe after we return.
> 
> device_register(&adapter->dev) calls device_initialize() and then
> device_add().
> 
> device_add() does a get_device(). device_add() also explicitly grabs
> the device's parent, and calls get_device() on it:
> 
>          parent = get_device(dev->parent);
> 
> So therefore, device_register() takes a lock on the parent PCI dev,
> which is what pci_dev_get() was guarding. pci_dev_get() can therefore
> be safely removed.
> 
> Fixes: f204e0b8cedd ("cxl: Driver code for powernv PCIe based cards for userspace access")
> Cc: stable@vger.kernel.org
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> Acked-by: Ian Munsie <imunsie@au1.ibm.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/2925c2fdf1e0eb642482f5b3

cheers