[3.16.y-ckt stable] Patch "cxl: Fix unbalanced pci_dev_get in cxl_probe" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    cxl: Fix unbalanced pci_dev_get in cxl_probe

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.19.8-ckt9.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From 62414d49c4c31a0a2e82c17a0b2c0e08514c7e85 Mon Sep 17 00:00:00 2001
From: Daniel Axtens <dja at axtens.net>
Date: Tue, 15 Sep 2015 15:04:07 +1000
Subject: cxl: Fix unbalanced pci_dev_get in cxl_probe

commit 2925c2fdf1e0eb642482f5b30577e9435aaa8edb upstream.

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")
Signed-off-by: Daniel Axtens <dja at axtens.net>
Acked-by: Ian Munsie <imunsie at au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 eee4fd6..cc55691 100644
--- a/drivers/misc/cxl/pci.c
+++ b/drivers/misc/cxl/pci.c
@@ -987,8 +987,6 @@ 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);

--
1.9.1