Subject: Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
From: Steven Rostedt <rostedt@goodmis.org>
To: Andrew Morton <akpm@osdl.org>
Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, matthew@wil.cx,
       arjan@infradead.org, torvalds@osdl.org, hch@infradead.org,
       mingo@elte.hu, pj@sgi.com, alan@lxorguk.ukuu.org.uk, tglx@linutronix.de,
       lkml@rtr.ca, dhowells@redhat.com
In-Reply-To: <20051215121817.2abb0166.akpm@osdl.org>
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Date: Thu, 15 Dec 2005 16:28:15 -0500
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On Thu, 2005-12-15 at 12:18 -0800, Andrew Morton wrote:
> Andrew Morton <akpm@osdl.org> wrote:
> >
> > David Howells <dhowells@redhat.com> wrote:
> > >
> > > So... Would you then object to an implementation of a mutex appearing in the
> > >  tree which semaphores that are being used as strict mutexes can be migrated
> > >  over to as the opportunity arises?
> > 
> > That would be sane.
> >
> 
> But not very.
> 
> Look at it from the POV of major architectures: there's no way the new
> mutex code will be faster than down() and up(), so we're adding a bunch of
> new tricky locking code which bloats the kernel and has to be understood
> and debugged for no gain.

I see it as a stepping stone for RT ;)

> 
> And I don't buy the debuggability argument really.  It'd be pretty simple
> to add debug code to the existing semaphore code to trap non-mutex usages. 
> Then go through the few valid non-mutex users and do:
> 
> #if debug
> 	sem->this_is_not_a_mutex = 1;
> #endif

That just looks plain ugly.  Still, if you want to keep the major archs
unchanged (at least until RT is in!) then just add the following:

#define mutex_lock(x) down(x)
#define mutex_unlock(x) up(x)
#define mutex_trylock(x) (!down_trylock(x))  /* see previous email! */

Then you can add your ugly patch ;) where on debug we define those
declared with DEFINE_SEM(x) add the this_is_not_a_mutex = 1

-- Steve


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