drm/xe/reg_sr: Remove register pool

That pool implementation doesn't really work: if the krealloc happens to
move the memory and return another address, the entries in the xarray
become invalid, leading to use-after-free later:

BUG: KASAN: slab-use-after-free in xe_reg_sr_apply_mmio+0x570/0x760 [xe]
Read of size 4 at addr ffff8881244b2590 by task modprobe/2753

Allocated by task 2753:
kasan_save_stack+0x39/0x70
kasan_save_track+0x14/0x40
kasan_save_alloc_info+0x37/0x60
__kasan_kmalloc+0xc3/0xd0
__kmalloc_node_track_caller_noprof+0x200/0x6d0
krealloc_noprof+0x229/0x380

Simplify the code to fix the bug. A better pooling strategy may be added
back later if needed.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Reviewed-by: Matt Roper <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Lucas De Marchi <[e-mail]>

drivers/gpu/drm/xe/xe_reg_sr.c

···
 #include "xe_reg_whitelist.h"
 #include "xe_rtp_types.h"
 
-#define XE_REG_SR_GROW_STEP_DEFAULT	16
-
 static void reg_sr_fini(struct drm_device *drm, void *arg)
 {
 	struct xe_reg_sr *sr = arg;
+	struct xe_reg_sr_entry *entry;
+	unsigned long reg;
+
+	xa_for_each(&sr->xa, reg, entry)
+		kfree(entry);
 
 	xa_destroy(&sr->xa);
-	kfree(sr->pool.arr);
-	memset(&sr->pool, 0, sizeof(sr->pool));
 }
 
 int xe_reg_sr_init(struct xe_reg_sr *sr, const char *name, struct xe_device *xe)
 {
 	xa_init(&sr->xa);
-	memset(&sr->pool, 0, sizeof(sr->pool));
-	sr->pool.grow_step = XE_REG_SR_GROW_STEP_DEFAULT;
 	sr->name = name;
 
 	return drmm_add_action_or_reset(&xe->drm, reg_sr_fini, sr);
 }
 EXPORT_SYMBOL_IF_KUNIT(xe_reg_sr_init);
-
-static struct xe_reg_sr_entry *alloc_entry(struct xe_reg_sr *sr)
-{
-	if (sr->pool.used == sr->pool.allocated) {
-		struct xe_reg_sr_entry *arr;
-
-		arr = krealloc_array(sr->pool.arr,
-				     ALIGN(sr->pool.allocated + 1, sr->pool.grow_step),
-				     sizeof(*arr), GFP_KERNEL);
-		if (!arr)
-			return NULL;
-
-		sr->pool.arr = arr;
-		sr->pool.allocated += sr->pool.grow_step;
-	}
-
-	return &sr->pool.arr[sr->pool.used++];
-}
 
 static bool compatible_entries(const struct xe_reg_sr_entry *e1,
 			       const struct xe_reg_sr_entry *e2)
···
 		return 0;
 	}
 
-	pentry = alloc_entry(sr);
+	pentry = kmalloc(sizeof(*pentry), GFP_KERNEL);
 	if (!pentry) {
 		ret = -ENOMEM;
 		goto fail;

drivers/gpu/drm/xe/xe_reg_sr_types.h

···
 };
 
 struct xe_reg_sr {
-	struct {
-		struct xe_reg_sr_entry *arr;
-		unsigned int used;
-		unsigned int allocated;
-		unsigned int grow_step;
-	} pool;
 	struct xarray xa;
 	const char *name;