Some more for program portability issues of MIPS processors.

 > I guess it would make no porting trouble if s_lock.c had
 > special statements like asm (".set mips2") inside.

As one of most successful RISC porcessors of the world, MIPS
processors do have instruction variations across range of
implementations.  Unfornately, the notion of instruction set
group (ISA, Instruction Set Architecture) which was originally
intended to categorize application program binary _natures_
are not useful nor even practical since processor
implementation diversity goes far beyond ISA category.  So,
ISA is a marketing term to name the products.

In this specific issue on LL/SC, program to use them should
have specific statements (pseudo assembler instructions)
to make clear some codes are extended instructions. It's
a simple practice for ease of portability, I believe in.

Toru Nishimura