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Date: Tue, 20 Dec 2005 14:43:30 -0500 (EST)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Russell King <rmk+lkml@arm.linux.org.uk>
cc: Nicolas Pitre <nico@cam.org>, Nick Piggin <nickpiggin@yahoo.com.au>,
       Ingo Molnar <mingo@elte.hu>, David Woodhouse <dwmw2@infradead.org>,
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Subject: Re: [patch 04/15] Generic Mutex Subsystem, add-atomic-call-func-x86_64.patch
In-Reply-To: <20051220192018.GB24199@flint.arm.linux.org.uk>
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On Tue, 20 Dec 2005, Russell King wrote:

> On Tue, Dec 20, 2005 at 11:35:22AM -0500, Nicolas Pitre wrote:
> > So 14 instructions total with preemption disabling, and that's with the
> > best implementation possible by open coding everything instead of
> > relying on generic functions/macros.
>
> I agree with your analysis Nicolas.
>
> However, don't forget to compare this with our existing semaphore
> implementation which is 9 instructions, or 8 for the SMP version.
>
> In total, this means that mutexes will be _FAR MORE EXPENSIVE_ on ARM
> than our semaphores.
>
> Forcing architectures down the "lets make everything generic" path
> does not always hack it.  It can't do by its very nature.  Generic
> implementations are *always* sub-optimal and it is pretty clear
> that any gain that mutexes _may_ give is completely wasted on ARM
> by the overhead of having a generic framework imposed upon us.
>
> So, to sum up, if this path is persued, mutexes will be a bloody
> disaster on ARM.  We'd be far better off sticking to semaphores
> and ignoring this mutex stuff.
>

So what's wrong with having the generic code, and for those with a fast
semapore add an arch specific?

#define mutex_lock down
#define mutex_unlock up
#define mutex_trylock(x) (!down_trylock(x))

Until the mutex code is updated to a fast arch specific implementation.

Let me restate, that the generic code should not be this, but each arch
can have this if they already went through great lengths in making a fast
semaphore.

Heck put the above defines in the generic code, with a define

linux/mutex.h:

#ifdef HAVE_ARCH_MUTEX
#include <asm/mutex.h>
#else

#ifdef HAVE_FAST_SEMAPHORE

#define <defines here>

#else

generic code here

#endif /* HAVE_FAST_SEMAPHORE */

#endif /* HAVE_ARCH_MUTEX */

-- Steve

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