Re: [patch 04/15] Generic Mutex Subsystem, add-atomic-call-func-x86_64.patch

[Russell King <rmk+lkml@arm.linux.org.uk>]

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.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core
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