Re: [PATCH] ath10k: prevent sta pointer rcu violation

[Johannes Berg <johannes@sipsolutions.net>]

> Unless you then continue to use that sta pointer after you release
> data_lock.

Ouch, ok. That's rather strangely hidden though.

> Consider this:
> 
> >          CPU0             CPU1
> > 1   synchronize_net()
> > 2    drv_sta_state()
> > 3                      htt_fetch_ind(pid,tid) called
> > 4                      rcu_read_lock()
> > 5                      get(data_lock)
> > 6                      txq=peers[pid]->sta->txq[tid]
> > 7                      put(data_lock)
> > 8    get(data_lock)
> > 9     peer->sta=0
> > 10   put(data_lock)
> > 11     kfree(sta)
> > 12                     ieee80211_tx_dequeue(txq)
> 
> Even though there's no code like (9) per se you can think of it as
> anything that tries to "remove" the peer--sta association
> (ath10k_peer
> is removed implicitly via wmi peer delete command and waiting for htt
> event completion).
> 
> Holding data_lock for the entire duration of handling fetch
> indication isn't really good for performance so it's better to fix
> RCU handling.

Yeah, I see it now - also the comment where this happens. You probably
should mark some things __rcu though and actually use RCU primitives,
so the code is actually understandable :)

johannes

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