[PATCH] rfkill: fix deadlock in rfkill_send_events

[Edward AD]

syzbot report:
syz-executor675/5132 is trying to acquire lock:
ffff8880297ee088 (&data->mtx){+.+.}-{3:3}, at: rfkill_send_events+0x226/0x3f0 net/rfkill/core.c:286

but task is already holding lock:
ffff88801bfc0088 (&data->mtx){+.+.}-{3:3}, at: rfkill_fop_open+0x146/0x750 net/rfkill/core.c:1183

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&data->mtx);
  lock(&data->mtx);

 *** DEADLOCK ***

In 2c3dfba4cf84 insert rfkill_sync() to rfkill_fop_open(), it will call
rfkill_send_events() and then triger this issue.

Fixes: 2c3dfba4cf84 ("rfkill: sync before userspace visibility/changes")
Reported-and-tested-by: syzbot+509238e523e032442b80@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Edward AD <twuufnxlz@xxxxxxxxx>
---
 net/rfkill/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index 08630896b6c8..a14e0d4a0b00 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1180,7 +1180,6 @@ static int rfkill_fop_open(struct inode *inode, struct file *file)
 	init_waitqueue_head(&data->read_wait);
 
 	mutex_lock(&rfkill_global_mutex);
-	mutex_lock(&data->mtx);
 	/*
 	 * start getting events from elsewhere but hold mtx to get
 	 * startup events added first
@@ -1191,9 +1190,12 @@ static int rfkill_fop_open(struct inode *inode, struct file *file)
 		if (!ev)
 			goto free;
 		rfkill_sync(rfkill);
+		mutex_lock(&data->mtx);
 		rfkill_fill_event(&ev->ev, rfkill, RFKILL_OP_ADD);
 		list_add_tail(&ev->list, &data->events);
+		mutex_unlock(&data->mtx);
 	}
+	mutex_lock(&data->mtx);
 	list_add(&data->list, &rfkill_fds);
 	mutex_unlock(&data->mtx);
 	mutex_unlock(&rfkill_global_mutex);
-- 
2.25.1