Re: [syzbot] INFO: task hung in rfkill_sync_work

[Magnus Leatti]

Hi,

This looks like the device_lock vs rfkill_global_mutex inversion
fixed by 1ab526d97a57 ("net: nfc: fix deadlock between
nfc_unregister_device and rfkill_fop_write"). That fix credited a
different syzbot report, so this one was never linked to it.

The report shows one task holding rfkill_global_mutex in
rfkill_fop_write and waiting for the NFC device lock in nfc_dev_down,
while another holds that device lock in nfc_unregister_device and
waits in rfkill_unregister. rfkill_sync_work is stuck behind them.

The C reproducer, 600 s per kernel, x86_64 under QEMU/KVM:

  1ab526d97a57^  hung task report at 60 s
  1ab526d97a57   no hang
  9d66f26fefac   no hang (mainline)

#syz fix: net: nfc: fix deadlock between nfc_unregister_device and rfkill_fop_write

Thanks,
Magnus