EDAC/ghes: Do not warn when incrementing refcount on 0

The following warning from the refcount framework is seen during ghes
initialization:

  EDAC MC0: Giving out device to module ghes_edac.c controller ghes_edac: DEV ghes (INTERRUPT)
  ------------[ cut here ]------------
  refcount_t: increment on 0; use-after-free.
  WARNING: CPU: 36 PID: 1 at lib/refcount.c:156 refcount_inc_checked
 [...]
  Call trace:
   refcount_inc_checked
   ghes_edac_register
   ghes_probe
   ...

It warns if the refcount is incremented from zero. This warning is
reasonable as a kernel object is typically created with a refcount of
one and freed once the refcount is zero. Afterwards the object would be
"used-after-free".

For GHES, the refcount is initialized with zero, and that is why this
message is seen when initializing the first instance. However, whenever
the refcount is zero, the device will be allocated and registered. Since
the ghes_reg_mutex protects the refcount and serializes allocation and
freeing of ghes devices, a use-after-free cannot happen here.

Instead of using refcount_inc() for the first instance, use
refcount_set(). This can be used here because the refcount is zero at
this point and can not change due to its protection by the mutex.

Fixes: 23f61b9f ("EDAC/ghes: Fix locking and memory barrier issues")
Reported-by: John Garry <john.garry@huawei.com>
Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: John Garry <john.garry@huawei.com>
Cc: <huangming23@huawei.com>
Cc: James Morse <james.morse@arm.com>
Cc: <linuxarm@huawei.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: <tanxiaofei@huawei.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: <wanghuiqiang@huawei.com>
Link: https://lkml.kernel.org/r/20191121213628.21244-1-rrichter@marvell.com

drivers/edac/ghes_edac.c

@@ -556,8 +556,8 @@ int ghes_edac_register(struct ghes *ghes, struct device *dev)
 	ghes_pvt = pvt;
 	spin_unlock_irqrestore(&ghes_lock, flags);
 
-	/* only increment on success */
-	refcount_inc(&ghes_refcount);
+	/* only set on success */
+	refcount_set(&ghes_refcount, 1);
 
 unlock:
 	mutex_unlock(&ghes_reg_mutex);