Re: [KJ] the rules for replacing DECLARE_MUTEX with DEFINE_MUTEX?

[Yoann Padioleau <padator@wanadoo.fr>]

"Robert P. J. Day" <rpjday@mindspring.com> writes:

>   i know that there's been a fair amount of work done lately in terms
> of replacing regular semaphores with the newer "mutex", and i just
> want to clarify under what circumstances that can be done.
>
>   first, a mutex is strictly a binary semaphore so it can't possibly
> be used in place of a general counting semaphore. (well, duh.)
>
>   in addition, mutexes can generally be used only in a limited
> locality, where the process that locks the mutex must be the same one
> that unlocks it, so even strictly binary semaphores might not be
> convertable to mutexes.

Why that ? The mutexes don't have a waiting queue like semaphores ? 

>
>   as i read it, the obvious places where actual mutexes could be used
> would be places like, say, drivers/spi/spi.c:
>
> ...
> static DECLARE_MUTEX(board_lock);
> ...
>     down(&board_lock);
>     list_add_tail(&bi->list, &board_list);
>     up(&board_lock);
> ...
>
>   first, the fact that the mutex itself is static means it's going to
> be used only within this file context.  

But it could be used by multiple instances of the same driver ? 

> and that restricted locality
> where it's actually used is what i interpret as a good hint that that
> semaphore could be replaced by a mutex.
>
>   is that a good summary of the rules?  am i missing anything
> critical?

Could you give counter examples: situations where we must not use 
a mutex, especially situations where it's a binary semaphore
but it still can't be converted to a mutex.

>
> rday
>
> p.s.  no, i have no interest in submitting patches for this, i just
> wanted to make sure i understood the general rules.

Just for curiosity, why you have no instest in such patches ? Because
you prefer to clarify the kernel than optimize it ? 


> -- 
> ====================================
> Robert P. J. Day
> Linux Consulting, Training and Annoying Kernel Pedantry
> Waterloo, Ontario, CANADA
>
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