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

[David Howells <dhowells@redhat.com>]

Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> Yes, the architecture code knows whether or not it implements atomic ops
> with spinlocks, so that architecture is in the position to decide to override
> the mutex implementation. *generic* code shouldn't worry about that, it should
> use the interfaces available, and if that isn't optimal on some architecture
> then that architecture can override it.

However, a number of generic templates can be provided if it makes things
easier for the arches because all they need to is:

	[arch/wibble/Kconfig]
	config MUTEX_TYPE_FOO
		bool
		default y

	[include/asm-wibble/system.h]
	#define __mutex_foo_this() { ... }
	#define __mutex_foo_that() { ... }

The unconditional two-state exchange I think will be a useful template for a
number of archs that don't have anything more advanced than XCHG/TAS/BSET/SWAP.

> It is not even clear that any ll/sc based architectures would need to override
> an atomic_cmpxchg variant at all because you can assume an unlocked fastpath

That's irrelevant. Any arch that has LL/SC almost certainly emulates CMPXCHG
with LL/SC.

> and not do the additional initial load to prime the cmpxchg.

Two points:

 (1) LL/SC does not require an additional initial load.

 (2) CMPXCHG does an implicit load; how else can it compare?

LL/SC can never be worse than CMPXCHG, if only because you're very unlikely to
have both, but it can be better.

David