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? > 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; >>
