linux-kernel - Re: initialize a mutex into locked state?

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Message-Id: <EDFA75CD-A3F5-4AA7-9FA2-7B7D61B6AAED@linuxhacker.ru>
Date:	Fri, 17 Jun 2016 10:24:32 -0400
From:	Oleg Drokin <green@...uxhacker.ru>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mailing List <linux-kernel@...r.kernel.org>, mingo@...hat.com,
	arjan@...ux.intel.com, "J . Bruce Fields" <bfields@...ldses.org>,
	Jeff Layton <jlayton@...chiereds.net>
Subject: Re: initialize a mutex into locked state?


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!

list_add(list, new_structure->s_list);
structure = new_structure;
out:
spin_unlock(somelock);
return structure;