Re: initialize a mutex into locked state?

[Jeff Layton <jlayton@poochiereds.net>]

On Fri, 2016-06-17 at 10:40 -0400, Oleg Drokin wrote:
> On Jun 17, 2016, at 10:32 AM, Peter Zijlstra wrote:
> 
> > 
> > On Fri, Jun 17, 2016 at 10:24:32AM -0400, Oleg Drokin wrote:
> > > 
> > > 
> > > On Jun 17, 2016, at 10:19 AM, Peter Zijlstra wrote:
> > > 
> > > > 
> > > > On Fri, Jun 17, 2016 at 10:14:10AM -0400, Oleg Drokin wrote:
> > > > > 
> > > > > 
> > > > > On Jun 17, 2016, at 4:25 AM, Peter Zijlstra wrote:
> > > > > 
> > > > > > 
> > > > > > On Wed, Jun 15, 2016 at 02:23:35PM -0400, Oleg Drokin
> > > > > > wrote:
> > > > > > > 
> > > > > > > Hello!
> > > > > > > 
> > > > > > > To my surprise I found out that it's not possible to
> > > > > > > initialise a mutex into
> > > > > > > a locked state.
> > > > > > > I discussed it with Arjan and apparently there's no
> > > > > > > fundamental reason
> > > > > > > not to allow this.
> > > > > > There is. A mutex _must_ have an owner. If you can
> > > > > > initialize it in
> > > > > > locked state, you could do so statically, ie. outside of
> > > > > > the context of
> > > > > > a task.
> > > > > What's wrong with disallowing only static initializers, but
> > > > > allowing dynamic ones?
> > > > > Then there is a clear owner.
> > > > At which point, what wrong with the simple:
> > > > 
> > > > 	mutex_init(&m);
> > > > 	mutex_lock(&m);
> > > > 
> > > > Sequence? Its obvious, has clear semantics and doesn't extend
> > > > the API.
> > > The problem is:
> > > 
> > > spin_lock(somelock);
> > > structure = some_internal_list_lookup(list);
> > > if (structure)
> > > 	goto out;
> > > 
> > > init_new_structure(new_structure);
> > > mutex_init(&new_structure->s_mutex);
> > > mutex_lock(&new_structure->s_mutex);  // XXX CANNOT DO THIS UNDER
> > > SPINLOCK!
> > 	mutex_trylock(&new_structure->s_mutex);
> > 
> > should work, since you know it cannot be acquired yet by anybody
> > else,
> > since you've not published it yet.
> This does work, but suddenly does not look so obvious anymore, does
> it?
> I got some feedback that doing this is not really preferred.
> 
> Also once __must_check is added to mutex_try_lock() (surprised it's
> not yet),
> we'll need to also have the useless "but what if it did fail to lock"
> path?
> 

Maybe just BUG in that case, and add a comment that says something
along the lines of "this should always work since it's not hashed yet"
?
 
> > 
> > And a trylock does not sleep, so is perfectly fine under a
> > spinlock.
> > 
> > > 
> > > 
> > > list_add(list, new_structure->s_list);
> > > structure = new_structure;
> > > out:
> > > spin_unlock(somelock);
> > > return structure;
> > > 

-- 
Jeff Layton <jlayton@poochiereds.net>