possible deadlock in handle_rx

[syzbot <syzbot+7f073540b1384a614e09@syzkaller.appspotmail.com>]

Hello,

syzbot hit the following crash on upstream commit
cb6416592bc2a8b731dabcec0d63cda270764fc6 (Sun Mar 25 17:45:10 2018 +0000)
Merge tag 'dmaengine-fix-4.16-rc7' of  
git://git.kernel.org/pub/scm/linux/kernel/git/vkoul/slave-dma
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=7f073540b1384a614e09

So far this crash happened 4 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=6506789075943424
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=5716250550337536
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5142038655795200
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-5034017172441945317
compiler: gcc (GCC) 7.1.1 20170620

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+7f073540b1384a614e09@syzkaller.appspotmail.com
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============================================
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


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