Date: Thu, 15 Dec 2005 12:18:17 -0800
From: Andrew Morton <akpm@osdl.org>
To: dhowells@redhat.com, lkml@rtr.ca, tglx@linutronix.de,
       alan@lxorguk.ukuu.org.uk, pj@sgi.com, mingo@elte.hu, hch@infradead.org,
       torvalds@osdl.org, arjan@infradead.org, matthew@wil.cx,
       linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
Message-Id: <20051215121817.2abb0166.akpm@osdl.org>
In-Reply-To: <20051215112855.31669dc1.akpm@osdl.org>
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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.

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
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