[coreboot-gerrit] Change in coreboot[master]: lib: Add a mutex

[Kyösti Mälkki (Code Review)]

Attention is currently required from: Raul Rangel, Julius Werner, Rob Barnes, Karthik Ramasubramanian.
Kyösti Mälkki has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/59320 )

Change subject: lib: Add a mutex
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Patch Set 6:

(2 comments)

File src/include/mutex.h:

https://review.coreboot.org/c/coreboot/+/59320/comment/e7f9117f_f72a440d 
PS6, Line 32: }
I put the following in <rules.h>

#if ENV_X86
   #define STAGE_HAS_SPINLOCKS		!ENV_ROMSTAGE_OR_BEFORE
   #elif ENV_RISCV
   #define STAGE_HAS_SPINLOCKS		1
   #else
   #define STAGE_HAS_SPINLOCKS		0
   #endif

/* When set <arch/smp/spinlock.h> is 
      included for the spinlock implementation. */
   #define ENV_STAGE_SUPPORTS_SMP	(CONFIG(SMP) && STAGE_HAS_SPINLOCKS)

The implementation of spinlock needed .data while mutex can do with .bss?
With that taken into account, you would never call the non-smp variants _mutex_lock_coop() or _mutex_unlock_coop() since romstage would also see ENV_STAGE_SUPPORTS_SMP=y ?

File src/lib/mutex.c:

https://review.coreboot.org/c/coreboot/+/59320/comment/9fcfa56c_42563eea 
PS6, Line 23: 			if (thread_yield() < 0) {
This path already handles COOP_MULTITHREADING=y, do you really want a separate _mutex_lock_coop() below that does not quarantee atomicity?