Questions about locking; turnstiles and sleeping threads

[John Baldwin]

On Thursday, November 13, 2014 4:52:50 am Adrian Chadd wrote:
> Hm, the more I dig into this, the more I realise it's not a 1:45am
> question to ask.
> 
> Specifically, callout_stop_safe() takes 'safe', which says "are we
> waiting around for this callout to finish if it started". Ie,
> callout_drain() is callout_stop_safe(c, 1) ; callout_stop() is
> callout_stop_safe(c, 0).
> 
> If safe is 1, then it'll potentially put the current thread to sleep
> in order to wait for it to synchronise with the callout that's
> running. It's sleeping with cc_lock which is the per-callwheel lock
> and it's doing that with whatever other locks are held. That's the
> situation which is tripping things up.
> 
> The manpage says that no locks should be held that the callout may
> block on, which isn't the case here at all - I'm trying to grab a lock
> in another thread that the caller _into_ the callout subsystem holds.
> The manpage doesn't mention anything about this. Sniffle.

It should just say "no sleepable locks at all".  And yes, callout_stop() is 
perfectly fine to call with locks held.  It is only callout_drain() that
should not be called, same as with bus_teardown_intr() and taskqueue_drain()
(other routines that can sleep while ensuring that an asynchronous task run
by another thread is stopped).

-- 
John Baldwin