Messages in this thread | | | Date | Fri, 17 Jun 2016 16:32:00 +0200 | From | Peter Zijlstra <> | Subject | Re: initialize a mutex into locked state? |
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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.
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; >
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