In procedures mlx4_cmd_use_events() and mlx4_cmd_use_polling(), we need to
guarantee that there are no FW commands in progress on the comm channel
(for VFs) or wrapped FW commands (on the PF) when SRIOV is active.

We do this by also taking the slave_cmd_mutex when SRIOV is active.

This is especially important when switching from event to polling, since we
free the command-context array during the switch.  If there are FW commands
in progress (e.g., waiting for a completion event), the completion event
handler will access freed memory.

Since the decision to use comm_wait or comm_poll is taken before grabbing
the event_sem/poll_sem in mlx4_comm_cmd_wait/poll, we must take the
slave_cmd_mutex as well (to guarantee that the decision to use events or
polling and the call to the appropriate cmd function are atomic).

Fixes: a7e1f049 ("net/mlx4_core: Fix deadlock when switching between polling and event fw commands")
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>