Questions about locking; turnstiles and sleeping threads

[Alexander Kabaev]

On Wed, 12 Nov 2014 18:13:55 -0800
Adrian Chadd <adrian at freebsd.org> wrote:

> Hi,
> 
> I have a bit of an odd case here.
> 
> I'm getting panics in the net80211/ath code, "sleeping thread (X) owns
> non-sleepable lock."
> 
> show alllocks just showed one lock held - the net80211 comlock. It's a
> recursive mutex, that's supposed to be sleepable.
> 
> The two threads in question look like this:
> 
> thread X: net80211_newstate_cb (grabs IEEE80211_LOCK())
>     ath_newstate
>     callout_drain - which grabs the ATH_LOCK as part of the callout
> drain side of things
>     that enters sleepq_wait() and goes to sleep, waiting for
> whatever's running the callout to
>     finish
> 
> thread Y:
>     rx_path in if_ath_rx_edma
>     ath_rx_pkt -> sta_input -> ath_recv_mgmt -> sta_recv_mgmt (grabs
> IEEE80211_LOCK()) -> panics
> 
> Thread Y doesn't hold any other locks. It's just trying to grab the
> IEEE80211_LOCK that is being held by thread X. But thread X is asleep
> waiting for whatever callout to finish so it can continue. The code in
> propagate_priority() sees that thread X is sleeping and panics.
> 
> So, what's really going on? I don't mind (well, "don't mind") having
> to take another deep dive through all of this to sort it out so it
> doesn't tickle the callout / turnstile code in this particular
> fashion, but I'd first like to ensure that it's not some corner case
> that isn't handled by the check in propagate_priority().
> 
> Thanks,
> 
> 
> -adrian
> _______________________________________________

Hi,

mutexes are blocking and not sleepable primitives, so doing any
unbounded sleep with mutex locked, such as one you are attempting by
calling callout_drain is illegal. In other words, you are getting an
expected assert and the code in question is wrong.


-- 
Alexander Kabaev
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