hwrng: core - start and stop in-kernel rngd in separate function

Extract the start/stop logic for the in-kernel rngd thread to
a separate function.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/core.c

@@ -51,7 +51,7 @@ MODULE_PARM_DESC(default_quality,
 
 static void drop_current_rng(void);
 static int hwrng_init(struct hwrng *rng);
-static void start_khwrngd(void);
+static void hwrng_manage_rngd(void);
 
 static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
 			       int wait);
@@ -164,10 +164,7 @@ static int hwrng_init(struct hwrng *rng)
 	if (current_quality > 1024)
 		current_quality = 1024;
 
-	if (current_quality == 0 && hwrng_fill)
-		kthread_stop(hwrng_fill);
-	if (current_quality > 0 && !hwrng_fill)
-		start_khwrngd();
+	hwrng_manage_rngd();
 
 	return 0;
 }
@@ -466,13 +463,20 @@ static int hwrng_fillfn(void *unused)
 	return 0;
 }
 
-static void start_khwrngd(void)
+static void hwrng_manage_rngd(void)
 {
+	if (WARN_ON(!mutex_is_locked(&rng_mutex)))
+		return;
+
+	if (current_quality == 0 && hwrng_fill)
+		kthread_stop(hwrng_fill);
+	if (current_quality > 0 && !hwrng_fill) {
 		hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
 		if (IS_ERR(hwrng_fill)) {
 			pr_err("hwrng_fill thread creation failed\n");
 			hwrng_fill = NULL;
 		}
+	}
 }
 
 int hwrng_register(struct hwrng *rng)