dma-mapping: benchmark: fix up kthread-related error handling

kthread creation failure is invalidly handled inside do_map_benchmark().
The put_task_struct() calls on the error path are supposed to balance the
get_task_struct() calls which only happen after all the kthreads are
successfully created. Rollback using kthread_stop() for already created
kthreads in case of such failure.

In normal situation call kthread_stop_put() to gracefully stop kthreads
and put their task refcounts. This should be done for all started
kthreads.

Found by Linux Verification Center (linuxtesting.org).

Fixes: 65789daa ("dma-mapping: add benchmark support for streaming DMA APIs")
Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/map_benchmark.c

@@ -118,6 +118,8 @@ static int do_map_benchmark(struct map_benchmark_data *map)
 		if (IS_ERR(tsk[i])) {
 			pr_err("create dma_map thread failed\n");
 			ret = PTR_ERR(tsk[i]);
+			while (--i >= 0)
+				kthread_stop(tsk[i]);
 			goto out;
 		}
 
@@ -139,13 +141,17 @@ static int do_map_benchmark(struct map_benchmark_data *map)
 
 	msleep_interruptible(map->bparam.seconds * 1000);
 
-	/* wait for the completion of benchmark threads */
+	/* wait for the completion of all started benchmark threads */
 	for (i = 0; i < threads; i++) {
-		ret = kthread_stop(tsk[i]);
-		if (ret)
-			goto out;
+		int kthread_ret = kthread_stop_put(tsk[i]);
+
+		if (kthread_ret)
+			ret = kthread_ret;
 	}
 
+	if (ret)
+		goto out;
+
 	loops = atomic64_read(&map->loops);
 	if (likely(loops > 0)) {
 		u64 map_variance, unmap_variance;
@@ -170,8 +176,6 @@ static int do_map_benchmark(struct map_benchmark_data *map)
 	}
 
 out:
-	for (i = 0; i < threads; i++)
-		put_task_struct(tsk[i]);
 	put_device(map->dev);
 	kfree(tsk);
 	return ret;