Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation

Russell King
2005-12-16T13:22:03Z

On Sat, Dec 17, 2005 at 12:01:27AM +1100, Nick Piggin wrote:
> You were proposing a worse default, which is the reason I suggested it.

I'd like to qualify that. "for architectures with native cmpxchg".

For general consumption (not specifically related to mutex stuff)...

For architectures with llsc, sequences stuch as:

load
modify
cmpxchg

are inefficient because they have to be implemented as:

load
modify
load
compare
store conditional

Now, if we consider using llsc as the basis of atomic operations:

load
modify
store conditional

and for cmpxchg-based architectures:

load
modify
cmpxchg

Notice that the cmpxchg-based case does _not_ get any worse - in fact
it's exactly identical. Note, however, that the llsc case becomes
more efficient.

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