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Date: Sat, 21 Apr 2001 01:53:42 +0200 (CEST)
From: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
To: Dennis <dennis@etinc.com>
cc: Matti Aarnio <matti.aarnio@zmailer.org>, linux-kernel@vger.kernel.org
Subject: Re: SMP in 2.4
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> 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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