i40e: avoid race condition when sending filters to firmware for addition

i40e: avoid race condition when sending filters to firmware for addition

Refactor how we add new filters to firmware to avoid a race condition
that can occur due to removing filters from the hash temporarily.

To understand the race condition, suppose that you have a number of MAC
filters, but have not yet added any VLANs. Now, add two VLANs in rapid
succession. A possible resulting flow would look something like the
following:

(1) lock hash for add VLAN
(2) add the new MAC/VLAN combos for each current MAC filter
(3) unlock hash
(4) lock hash for filter sync
(5) notice that we have a VLAN, so prepare to update all MAC filters
    with VLAN=-1 to be VLAN=0.
(6) move NEW and REMOVE filters to temporary list
(7) unlock hash
(8) lock hash for add VLAN
(9) add new MAC/VLAN combos. Notice that no MAC filters are currently in
    the hash list, so we don't add any VLANs <--- BUG!
(10) unlock hash
(11) sync the temporary lists to firmware
(12) lock hash for post-sync
(13) move the temporary elements back to the main list
....

Because we take filters out of the main hash into temporary lists, we
introduce a narrow window where it is possible that other callers to the
list will not see some of the filters which were previously added but
have not yet been finalized. This results in sometimes dropping VLAN
additions, and could also result in failing to add a MAC address on the
newly added VLAN.

One obvious way to avoid this race condition would be to lock the entire
firmware process. Unfortunately this does not work because adminq
firmware commands take a mutex which results in a sleep while atomic
BUG(). So, we can't use the simplest approach.

An alternative approach is to simply not remove the filters from the
hash list while adding. Instead, add an i40e_new_mac_filter structure
which we will use to track added filters. This avoids the need to remove
the filter from the hash list. We'll store a pointer to the original
i40e_mac_filter, along with our own copy of the state.

We won't update the state directly, so as to avoid race with other code
that may modify the state while under the lock. We are safe to read
f->macaddr and f->vlan since these only change in two locations. The
first is on filter creation, which must have already occurred. The
second is inside i40e_correct_vlan_filters which was previously run
after creation of this object and can't be run again until after. Thus,
we should be safe to read the MAC address and VLAN while outside the
lock.

We also aren't going to run into a use-after-free issue because the only
place where we free filters is when they are marked FAILED or when we
remove them inside the sync subtask. Since the subtask has its own
critical flag to prevent duplicate runs, we know this won't happen. We
also know that the only location to transition a filter from NEW to
FAILED is inside the subtask also, so we aren't worried about that
either.

Use the wrapper i40e_new_mac_filter for additions, and once we've
finalized the addition to firmware, we will update the filter state
inside a lock, and then free the wrapper structure.

In order to avoid a possible race condition with filter deletion, we
won't update the original filter state unless it is still
I40E_FILTER_NEW when we finish the firmware sync.

This approach is more complex, but avoids race conditions related to
filters being temporarily removed from the list. We do not need the same
behavior for deletion because we always unconditionally removed the
filters from the list regardless of the firmware status.

Change-Id: I14b74bc2301f8e69433fbe77ebca532db20c5317
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>