drm/i915/gtt: Handle double alloc failures

Matthew pointed out that we could face a double failure with concurrent
allocations/frees, and so the assumption that the local var alloc was
NULL was fraught with danger. Rather than complicate the error paths too
much to add a second local for a second free, just do the second free
earlier on the unwind path.

Reported-by: Matthew Auld <matthew.william.auld@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190704104345.6603-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1484,6 +1484,10 @@ static int gen8_ppgtt_alloc_pdp(struct i915_address_space *vm,
 	goto out;
 
 unwind_pd:
+	if (alloc) {
+		free_pd(vm, alloc);
+		alloc = NULL;
+	}
 	spin_lock(&pdp->lock);
 	if (atomic_dec_and_test(&pd->used)) {
 		gen8_ppgtt_set_pdpe(pdp, vm->scratch_pd, pdpe);
@@ -1556,6 +1560,10 @@ static int gen8_ppgtt_alloc_4lvl(struct i915_address_space *vm,
 	goto out;
 
 unwind_pdp:
+	if (alloc) {
+		free_pd(vm, alloc);
+		alloc = NULL;
+	}
 	spin_lock(&pml4->lock);
 	if (atomic_dec_and_test(&pdp->used)) {
 		gen8_ppgtt_set_pml4e(pml4, vm->scratch_pdp, pml4e);