Questions about locking; turnstiles and sleeping threads

[Adrian Chadd]

On 13 November 2014 06:48, John Baldwin <jhb at freebsd.org> wrote:
> 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).

so, we should add WITNESS_WARN() to those as well?




-adrian