[RFT] net80211 TX serialisation, take #4

[Adrian Chadd]

On 23 February 2013 22:30, PseudoCylon <moonlightakkiy at yahoo.ca> wrote:
>> So, this is all pretty terrible. The only sane solution for now is to
>> make my VAP TX lock an IC TX lock,and grab said IC TX lock for all
>> VAPs. That way the driver can grab the IC TX lock when it's doing
>> deferred sends and it'll be sure the lock is held when it decides to
>> grab/increment sequence numbers from ni->ni_txseqs[].
>
> I don't think
>   lock();
>   ni->ni_txseqs[]++;
>   unlock();
> can fix the problem because no one can guarantee which process/thread
> grabs the lock next.

Yup, that's definitely a problem.

The problem here is that packets coming into net80211 - either via
vap->input() (bpf injection) or vap->transmit() (802.3) , have to do
this:

* processed in order;
* the same order as they're then pushed into the power save / age queue;
* the same order that these frames get assigned sequence numbers and
enforce state (eg considering things for ampdu, power save, etc);
* the same order that they're queued to the driver;
* the same order that they're dequeued from the driver;
* the same order that they're processed (eg crypto encaps);
* the same order that it's then passed down into the driver - either
via direct dispatch or deferred queue.


>> * .. and do this without tearing my hair out.
>
> The sequence will be messed up during moving packets from one queue to
> another, i.e from driver queue to hardware queue. As long as packets
> are in a queue (in a linked list) sequence won't change unless we
> explicitly write such code. So...
>
> Saving your hair option 1
> tx()
> {
>         for() {
>                 lock();
>                 dequeue(m);    /* assuming queue is in order */
>                 ni_txseqs[]++
>                 enqueue_working_queue(m);
>                 unlock();
>                 ...
>                 process m
>                 ...
>                 lock();
>                 /*
>                  * m may change here.
>                  * Whichever the thread does dequeues, m0 will be
>                  * the head of the queue, so sequence will be kept intact.
>                  * But, need to test if processing of m0 has been completed.
>                  */
>                 dequeue_working_queue(m0);
>                 enqueue_driver_queue(m0);  /* or hardware_queue() */
>                 unlock();
>         }
> }
> This will keep sequence intact.

Right. This is similar to my idea (or two.)

There's a few other issues though!

ic_raw_xmit() is called by a bunch of places to generate both raw
frames and management frames. This bypasses the vap queue and calls
the driver direct. As an example, injected EAPOL frames have CCMP IV
numbers (as they're encrypted!) but not necessarily sequence numbers.
Because the CCMP IV gets calculated int he driver at some point after
it's been queued, ANY slight race here causes some other frame to get
queued with a CCMP IV -after- the EAPOL frame, and it will get
dropped. Then you get your session dropped. :-)

ic_raw_xmit() is also an ic method, not a vap method. Yes, it bypasses
the vap queue.

There's deferred transmission going on (eg ath_start() getting called
from TX completion, as an example.) Should that be called under the
above lock() in your example?

And the TX sequence number stuff in iwn and ath, because the
aggregation code reuses the ni_txseqs[] array. So yes, the locking
wouldn't clash with the rest of net80211 (as only either the non-agg
TX path in net80211 or the TX path in ath/iwn would be using that
variable) but it's still a pain in the ass.

Grr.

The ridiculously complicated, broken-down-into-bits version:

* add a new mbuf tag to mbufs (optional) which lets us tag an mbuf
with a bpf TX params struct (ie, what gets optionally passed into the
raw xmit call);
* calls to ic_raw_xmit() become a VAP raw xmit;
* the VAP raw xmit pushes mbufs into the same ifnet queue that the VAP
frames have (ie right now the VAP ifnet queue) - ie, serialise the raw
and non-raw VAP frames before it gets processed for 802.11 state;
* .. then move the VAP transmit to your model - which is
  + vap transmit and raw transmit just push frames into the vap ifnet queue;
  + vap transmit then schedules a deferred task or a direct dispatch +
deferred task if we cant grab the VAP TX lock;
  + the vap transmit task/dispatch routine handles the frames one at a
time - either inside a task queue or inside the VAP TX lock - thus all
of the 802.3 -> 802.11 encapsulation and state handling is correctly
serialised _and_ the order of sequence numbers and frame handling is
maintained;
  + then those frames have to be queued to the driver in _exactly_ the
same order - so either the VAP TX task/process queues them into a
driver staging queue and kicks the driver side ,or it goes into a
separate task to do driver dispatch;
  + .. and then it's up to the driver to dequeue each frame, handle it
and push them into the hardware in the same order.

Now, that's totally, ridiculously complicated but it does avoid a
bunch of reentrant and long-held-lock problems.

The ridiculously uncomplicated version:

* put a big IC TX lock around ieee80211_start(), ieee80211_output()
and any routine that is calling ieee80211_encap() -> if_transmit() or
ieee80211_encap() -> ic_raw_xmit();
* do the same with the ageq and powersave queue that dispatches to the
driver - grab that big IC TX lock, dequeue all the frames to the
driver, release the IC lock;
* the driver TX code must grab the net80211 IC TX lock before it does
a deferred transmit - so the driver TX (not TX completion - TX!)
always has a consistent set of locking;
* do a bunch of net80211 TX path refactoring to unify the TX path a bit;
* just give in to the long-held locks and worry about tidying this
crap up later;
* hope to dear god there aren't any lock-order issues between the
comlock and this new lock; same as the other crap that's going on;

Opinions? :)



Adrian