Re: initialize a mutex into locked state?

Oleg Drokin
2016-06-17T14:40:54Z

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;
>>