The ghes registration and refcount is broken in several ways:

 * ghes_edac_register() returns with success for a 2nd instance
   even if a first instance's registration is still running. This is
   not correct as the first instance may fail later. A subsequent
   registration may not finish before the first. Parallel registrations
   must be avoided.

 * The refcount was increased even if a registration failed. This
   leads to stale counters preventing the device from being released.

 * The ghes refcount may not be decremented properly on unregistration.
   Always decrement the refcount once ghes_edac_unregister() is called to
   keep the refcount sane.

 * The ghes_pvt pointer is handed to the irq handler before registration
   finished.

 * The mci structure could be freed while the irq handler is running.

Fix this by adding a mutex to ghes_edac_register(). This mutex
serializes instances to register and unregister. The refcount is only
increased if the registration succeeded. This makes sure the refcount is
in a consistent state after registering or unregistering a device.

Note: A spinlock cannot be used here as the code section may sleep.

The ghes_pvt is protected by ghes_lock now. This ensures the pointer is
not updated before registration was finished or while the irq handler is
running. It is unset before unregistering the device including necessary
(implicit) memory barriers making the changes visible to other CPUs.
Thus, the device can not be used anymore by an interrupt.

Also, rename ghes_init to ghes_refcount for better readability and
switch to refcount API.

A refcount is needed because there can be multiple GHES structures being
defined (see ACPI 6.3 specification, 18.3.2.7 Generic Hardware Error
Source, "Some platforms may describe multiple Generic Hardware Error
Source structures with different notification types, ...").

Another approach to use the mci's device refcount (get_device()) and
have a release function does not work here. A release function will be
called only for device_release() with the last put_device() call. The
device must be deleted *before* that with device_del(). This is only
possible by maintaining an own refcount.

 [ bp: touchups. ]

Fixes: 0fe5f281 ("EDAC, ghes: Model a single, logical memory controller")
Fixes: 1e72e673 ("EDAC/ghes: Fix Use after free in ghes_edac remove path")
Co-developed-by: James Morse <james.morse@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Co-developed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20191105200732.3053-1-rrichter@marvell.com