usb: gadget: f_midi: Use refcount when freeing f_midi_opts

Currently, the midi function is not freed until it is
both removed from the config and released by the user.
Since the user could take a long time to release the
card, it's possible that the function could be unlinked
and thus f_midi_opts would be null when freeing f_midi.

Thus, refcount f_midi_opts and only free it when it is
unlinked and all f_midis have been freed.
Signed-off-by: Jerry Zhang <zhangjerry@google.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_midi.c

@@ -109,6 +109,7 @@ static inline struct f_midi *func_to_midi(struct usb_function *f)
 
 static void f_midi_transmit(struct f_midi *midi);
 static void f_midi_rmidi_free(struct snd_rawmidi *rmidi);
+static void f_midi_free_inst(struct usb_function_instance *f);
 
 DECLARE_UAC_AC_HEADER_DESCRIPTOR(1);
 DECLARE_USB_MIDI_OUT_JACK_DESCRIPTOR(1);
@@ -1102,7 +1103,7 @@ static ssize_t f_midi_opts_##name##_store(struct config_item *item,	\
 	u32 num;							\
 									\
 	mutex_lock(&opts->lock);					\
-	if (opts->refcnt) {						\
+	if (opts->refcnt > 1) {						\
 		ret = -EBUSY;						\
 		goto end;						\
 	}								\
@@ -1157,7 +1158,7 @@ static ssize_t f_midi_opts_id_store(struct config_item *item,
 	char *c;
 
 	mutex_lock(&opts->lock);
-	if (opts->refcnt) {
+	if (opts->refcnt > 1) {
 		ret = -EBUSY;
 		goto end;
 	}
@@ -1198,13 +1199,21 @@ static const struct config_item_type midi_func_type = {
 static void f_midi_free_inst(struct usb_function_instance *f)
 {
 	struct f_midi_opts *opts;
+	bool free = false;
 
 	opts = container_of(f, struct f_midi_opts, func_inst);
 
-	if (opts->id_allocated)
-		kfree(opts->id);
+	mutex_lock(&opts->lock);
+	if (!--opts->refcnt) {
+		free = true;
+	}
+	mutex_unlock(&opts->lock);
 
-	kfree(opts);
+	if (free) {
+		if (opts->id_allocated)
+			kfree(opts->id);
+		kfree(opts);
+	}
 }
 
 static struct usb_function_instance *f_midi_alloc_inst(void)
@@ -1223,6 +1232,7 @@ static struct usb_function_instance *f_midi_alloc_inst(void)
 	opts->qlen = 32;
 	opts->in_ports = 1;
 	opts->out_ports = 1;
+	opts->refcnt = 1;
 
 	config_group_init_type_name(&opts->func_inst.group, "",
 				    &midi_func_type);
@@ -1234,6 +1244,7 @@ static void f_midi_free(struct usb_function *f)
 {
 	struct f_midi *midi;
 	struct f_midi_opts *opts;
+	bool free = false;
 
 	midi = func_to_midi(f);
 	opts = container_of(f->fi, struct f_midi_opts, func_inst);
@@ -1242,9 +1253,12 @@ static void f_midi_free(struct usb_function *f)
 		kfree(midi->id);
 		kfifo_free(&midi->in_req_fifo);
 		kfree(midi);
-		--opts->refcnt;
+		free = true;
 	}
 	mutex_unlock(&opts->lock);
+
+	if (free)
+		f_midi_free_inst(&opts->func_inst);
 }
 
 static void f_midi_rmidi_free(struct snd_rawmidi *rmidi)