Re: SMP in 2.4

Mikulas Patocka (mikulas@artax.karlin.mff.cuni.cz)
Sat, 21 Apr 2001 01:53:42 +0200 (CEST)


> 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

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/