need for another mutex type/flag?

[Julian Elischer]

John Baldwin wrote:
> On Monday 26 January 2009 5:51:40 am Alfred Perlstein wrote:
>> * Julian Elischer <julian at elischer.org> [090125 22:53] wrote:
>>> Alfred Perlstein wrote:
>>>> Jeff, I think that Julian really wants to prevent a sleep inside
>>>> his context.  Right now, I think we only check for mutexes held
>>>> before a sleep that arne't sleepable.  It might make sense to allow
>>>> one to just mark a thread non-sleepable even though no mutex is
>>>> held.
>>>>
>>>> Julian, is that right?
>>> basically, though I don't know the details of implementation..
>>> I just know that mutexes per se aren't bad for netgraph but
>>> that node authors need some guidance on how to use them and
>>> some way to prove to them when they do the wrong thing.
>> The way to add the assertion you want would be to keep a count
>> inside of the thread structure "td_nosleep", set to 0 at thread
>> creation, then you can do this:
>>
>> TD_SLEEP_NO(td);  /* td->td_nosleep++ */
>> call_some_untrusted_code();
>> TD_SLEEP_OK(td);  /* td->td_nosleep-- */
>>
>> Then add this to subr_witness.c:witness_warn():
>>
>>    if (flags & WARN_SLEEPOK && td->td_nosleep != 0) {
>>       printf("Sleeping in unsleepable context.\n");
>>       n++;   /* this variable is local to witness_warn() 
>>                 and triggers an ASSERT at the end */
>>    }
>>
>> I could have sworn we already had such a feature, but it appears
>> that it's only accessable if you're holding a lock, if you added
>> this counter, then you could catch sleeps without needing a lock
>> held.
> 
> We have this feature already for sleeping, but I think Julian isn't worried 
> about sleeping (i.e. *sleep() or cv_*wait*()), but wants to prevent the code 
> from acquiring any other locks.  It's easy to add a MTX_LEAF, I'm just not 
> sure if we really want to micro-manage the code that much.  WITNESS will 
> already catch any LORs, and if they are acquiring a rarely-contested lock 
> then they aren't going to back up traffic in the common case.
> 

maybe what I want is to be able to label a lock as "fleeting"
By which I mean that the work that would be done while holding this 
lock would be fleeting (momentary) in nature.

An example f a fleeting lock would be something that gains the lock
in order to safely switch two pointers. no malloc is required and
nothing is going to take a long time.

locks that are NOT momentary include holding the process list lock 
while allocating a large buffer (series of them) and dumping the
contents of all processes and things like that.

one might almost say that a fleeting lock might be gotten while 
holding another fleeting lock, but that is pushing it for me..