Change in coreboot[master]: arch/x86/smp/spinlock: Fix threading when !STAGE_HAS_SPINLOCKS

[Raul Rangel (Code Review)]

Attention is currently required from: Furquan Shaikh, Tim Wawrzynczak, Julius Werner, Kyösti Mälkki, Rob Barnes, Karthik Ramasubramanian.
Raul Rangel has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/59021 )

Change subject: arch/x86/smp/spinlock: Fix threading when !STAGE_HAS_SPINLOCKS
......................................................................

Patch Set 2:

(1 comment)

File src/arch/x86/include/arch/smp/spinlock.h:

https://review.coreboot.org/c/coreboot/+/59021/comment/6b677db3_578465d7 
PS2, Line 50: 	thread_coop_disable();
> This function is attributed with __always_inline but got added a function call into it with CB:56320 […]

I can remove the __always_inline. Not really sure why we need it.

Yeah, if you have two threads trying to print things then we need to serialize the threads. If on the other hand one of the threads is using a DMA controller to perform some operation while the other thread is printing to the console then things are still working in parallel. This is the current goal of coop.

The other option is we add a mutex everywhere we have a spinlock. This doesn't really solve the problem though. We still can't yield while holding a spin lock, so we still need to disable threading.