Re: [PATCH iwl-net] e1000: Move cancel_work_sync to avoid deadlock

[Stanislav Fomichev <stfomichev@gmail.com>]

On 05/30, Joe Damato wrote:
> Previously, e1000_down called cancel_work_sync for the e1000 reset task
> (via e1000_down_and_stop), which takes RTNL.
> 
> As reported by users and syzbot, a deadlock is possible due to lock
> inversion in the following scenario:
> 
> CPU 0:
>   - RTNL is held
>   - e1000_close
>   - e1000_down
>   - cancel_work_sync (takes the work queue mutex)
>   - e1000_reset_task
> 
> CPU 1:
>   - process_one_work (takes the work queue mutex)
>   - e1000_reset_task (takes RTNL)

nit: as Jakub mentioned in another thread, it seems more about the
flush_work waiting for the reset_task to complete rather than
wq mutexes (which are fake)?

CPU 0:
  - RTNL is held
  - e1000_close
  - e1000_down
  - cancel_work_sync
  - __flush_work
  - <wait here for the reset_task to finish>

CPU 1:
  - process_one_work
  - e1000_reset_task (takes RTNL)
  - <but cpu 0 already holds rtnl>

The fix looks good!

Acked-by: Stanislav Fomichev <sdf@fomichev.me>