syzbot

============================================
WARNING: possible recursive locking detected
4.16.0-rc6+ #366 Not tainted
--------------------------------------------
vhost-4248/4760 is trying to acquire lock:
 (&vq->mutex){+.+.}, at: [<000000003482bddc>] vhost_net_rx_peek_head_len drivers/vhost/net.c:633 [inline]
 (&vq->mutex){+.+.}, at: [<000000003482bddc>] handle_rx+0xeb1/0x19c0 drivers/vhost/net.c:784

but task is already holding lock:
 (&vq->mutex){+.+.}, at: [<000000004de72f44>] handle_rx+0x1f5/0x19c0 drivers/vhost/net.c:766

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

       CPU0
       ----
  lock(&vq->mutex);
  lock(&vq->mutex);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

1 lock held by vhost-4248/4760:
 #0:  (&vq->mutex){+.+.}, at: [<000000004de72f44>] handle_rx+0x1f5/0x19c0 drivers/vhost/net.c:766

stack backtrace:
CPU: 0 PID: 4760 Comm: vhost-4248 Not tainted 4.16.0-rc6+ #366
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x24d lib/dump_stack.c:53
 print_deadlock_bug kernel/locking/lockdep.c:1761 [inline]
 check_deadlock kernel/locking/lockdep.c:1805 [inline]
 validate_chain kernel/locking/lockdep.c:2401 [inline]
 __lock_acquire+0xe8f/0x3e00 kernel/locking/lockdep.c:3431
 lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
 __mutex_lock_common kernel/locking/mutex.c:756 [inline]
 __mutex_lock+0x16f/0x1a80 kernel/locking/mutex.c:893
 mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
 vhost_net_rx_peek_head_len drivers/vhost/net.c:633 [inline]
 handle_rx+0xeb1/0x19c0 drivers/vhost/net.c:784
 handle_rx_net+0x19/0x20 drivers/vhost/net.c:910
 vhost_worker+0x268/0x470 drivers/vhost/vhost.c:361
 kthread+0x33c/0x400 kernel/kthread.c:238
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406