linux-kernel - Re: [RFC,PATCH] mutex: mutex_is

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Date:	Wed, 4 Nov 2009 16:40:15 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC,PATCH] mutex: mutex_is_owner() helper


* Eric Dumazet <eric.dumazet@...il.com> wrote:

> mutex_is_locked() is called most of the time to check if mutex is locked by current
> thread. But it's a lazy check, because mutex might be locked by another thread.
> 
> Adds a new mutex_is_owned_by() helper, that can check ownership if CONFIG_SMP or
> CONFIG_DEBUG_MUTEXES are set.
> 
> Returns are 
>  0 if mutex is unlocked.
>  1 if locked
> -1 if not locked by designated thread.
> 
> Last return value is possible only if CONFIG_SMP=y or CONFIG_DEBUG_MUTEXES=y
> 
> Example of use :
> 
> int rtnl_is_locked(void)
> {
> 	return mutex_is_locked(&rtnl_mutex);
> }
> ->
> int rtnl_is_locked(void)
> {
> 	return mutex_is_owned_by(&rtnl_mutex, current_thread_info()) == 1;
> }

To make sure this does not extend mutexes to be used a recursive 
mutexes, mind naming it more clearly, like debug_mutex_is_owned(), and 
adding a comment that says that this shouldnt be taken?

Also, it's somewhat imprecise: on !SMP && !DEBUG_MUTEXES we might return 
a false '1'. Which happens to work for the rtnl usecase - but might not 
in other cases.

	Ingo
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