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SubjectRe: SMP in 2.4
> I was referring to the infamous CLI/STI combinations that are more 
> analogous to spinlocks than anything you are talking about. spl levels are
> clean and transparent and have been doing a very nice job in helping to
> avoid race conditions in real unix systems for quite some time now.

It has nothing to do with smp ;)

spl levels are actually faster, because hardware interrupt locking
routines are poorly optimized in processors.

I looked at P-6 instruction timing table and found:

PUSHF 16 upos
POPF 17 uops
CLI 9 uops
STI 17 uops

I think soft interrupt locks like this would be better (at least on i386):

cli:
movb $0, intr_lock

sti:
movb $1, intr_lock
testb $1, intr_pending
jnz somewhere_away_to_handle_defered_interrupt

save_flags:
movb intr_lock, %al

restore_flags:
movb %al, intr_lock
testb %al, intr_pending
jnz somewhere_away_to_handle_defered_interrupt

And - of course - interrupt checks intr_lock in its entry and if it is
zero, sets intr_pending and exits immediatelly.

Mikulas

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