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Subjectpossible deadlock in handle_rx

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
syzbot dashboard link:

So far this crash happened 4 times on upstream.
C reproducer:
syzkaller reproducer:
Raw console output:
Kernel config:
compiler: gcc (GCC) 7.1.1 20170620

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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

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

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


*** 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

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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