misc: sgi-gru: Convert from atomic_t to refcount_t on gru_thread_state->ts_refcnt

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Acked-by: Dimitri Sivanich <sivanich@hpe.com>
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Link: https://lore.kernel.org/r/1626517043-42696-1-git-send-email-xiyuyang19@fudan.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/sgi-gru/grumain.c

@@ -282,7 +282,7 @@ static void gru_unload_mm_tracker(struct gru_state *gru,
  */
 void gts_drop(struct gru_thread_state *gts)
 {
-	if (gts && atomic_dec_return(&gts->ts_refcnt) == 0) {
+	if (gts && refcount_dec_and_test(&gts->ts_refcnt)) {
 		if (gts->ts_gms)
 			gru_drop_mmu_notifier(gts->ts_gms);
 		kfree(gts);
@@ -323,7 +323,7 @@ struct gru_thread_state *gru_alloc_gts(struct vm_area_struct *vma,
 
 	STAT(gts_alloc);
 	memset(gts, 0, sizeof(struct gru_thread_state)); /* zero out header */
-	atomic_set(&gts->ts_refcnt, 1);
+	refcount_set(&gts->ts_refcnt, 1);
 	mutex_init(&gts->ts_ctxlock);
 	gts->ts_cbr_au_count = cbr_au_count;
 	gts->ts_dsr_au_count = dsr_au_count;
@@ -888,7 +888,7 @@ again:
 		gts->ts_gru = gru;
 		gts->ts_blade = gru->gs_blade_id;
 		gts->ts_ctxnum = gru_assign_context_number(gru);
-		atomic_inc(&gts->ts_refcnt);
+		refcount_inc(&gts->ts_refcnt);
 		gru->gs_gts[gts->ts_ctxnum] = gts;
 		spin_unlock(&gru->gs_lock);
 

drivers/misc/sgi-gru/grutables.h

@@ -129,6 +129,7 @@
  *
  */
 
+#include <linux/refcount.h>
 #include <linux/rmap.h>
 #include <linux/interrupt.h>
 #include <linux/mutex.h>
@@ -358,7 +359,7 @@ struct gru_thread_state {
 						     enabled */
 	int			ts_ctxnum;	/* context number where the
 						   context is loaded */
-	atomic_t		ts_refcnt;	/* reference count GTS */
+	refcount_t		ts_refcnt;	/* reference count GTS */
 	unsigned char		ts_dsr_au_count;/* Number of DSR resources
 						   required for contest */
 	unsigned char		ts_cbr_au_count;/* Number of CBR resources