[PATCH v2] netdevsim: Fix deadlock in del_device_store() and nsim_bus_exit()

[Moksh Panicker <mokshpanicker.7@gmail.com>]

del_device_store() holds nsim_bus_dev_list_lock while calling
nsim_bus_dev_del(), which calls device_unregister() which internally
acquires the device lock. Similarly, nsim_bus_exit() holds the same
lock while calling nsim_bus_dev_del(). If another thread already holds
the device lock and tries to acquire nsim_bus_dev_list_lock, a deadlock
occurs:

  INFO: task hung in nsim_bus_dev_del

Fix this by releasing nsim_bus_dev_list_lock before calling
nsim_bus_dev_del() in both locations, after the devices have already
been removed from the list with list_del().

A similar issue exists in new_device_store() which can be addressed
separately.

Reported-by: syzbot+1cf303af03cf30b1275a@syzkaller.appspot.com
Closes: https://syzkaller.appspot.com/bug?extid=1cf303af03cf30b1275a
Signed-off-by: Moksh Panicker <mokshpanicker.7@gmail.com>
---
 drivers/net/netdevsim/bus.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/netdevsim/bus.c b/drivers/net/netdevsim/bus.c
index 41483e371..0f02ff8ad 100644
--- a/drivers/net/netdevsim/bus.c
+++ b/drivers/net/netdevsim/bus.c
@@ -241,11 +241,12 @@ del_device_store(const struct bus_type *bus, const char *buf, size_t count)
 		if (nsim_bus_dev->dev.id != id)
 			continue;
 		list_del(&nsim_bus_dev->list);
-		nsim_bus_dev_del(nsim_bus_dev);
 		err = 0;
 		break;
 	}
 	mutex_unlock(&nsim_bus_dev_list_lock);
+	if (!err)
+		nsim_bus_dev_del(nsim_bus_dev);
 	return !err ? count : err;
 }
 static BUS_ATTR_WO(del_device);
@@ -527,11 +528,11 @@ void nsim_bus_exit(void)
 		complete(&nsim_bus_devs_released);
 
 	mutex_lock(&nsim_bus_dev_list_lock);
-	list_for_each_entry_safe(nsim_bus_dev, tmp, &nsim_bus_dev_list, list) {
+	list_for_each_entry_safe(nsim_bus_dev, tmp, &nsim_bus_dev_list, list)
 		list_del(&nsim_bus_dev->list);
-		nsim_bus_dev_del(nsim_bus_dev);
-	}
 	mutex_unlock(&nsim_bus_dev_list_lock);
+	list_for_each_entry_safe(nsim_bus_dev, tmp, &nsim_bus_dev_list, list)
+		nsim_bus_dev_del(nsim_bus_dev);
 
 	wait_for_completion(&nsim_bus_devs_released);
 
-- 
2.34.1