Question and patch about spinlocks (x86)

[Mark Zealey <mark@zealos.org>]

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Just looking through at the spinlock assembly I noticed a few things which I
think are bugs:
 
 	"js 2f\n" \
 	".section .text.lock,\"ax\"\n" \
 	"2:\t" \
	"cmpb $0,%0\n\t" \
 	"rep;nop\n\t" \
	"jle 2b\n\t" \
 	"jmp 1b\n" \
 	".previous"

We do the cmp loop as a 'soft' check, as the lock operand locks the whole system
bus, stopping the system for a while (as much as 70 cycles, I believe). However,
I don't understand why it was put before the 'rep; nop' which just sets the
processor to wait for a bit. Surely it would be better to test *after* we have
waited, as then we have a better chance of it being correct.

Any comments? Attached is a patch to fix it.

-- 

Mark Zealey (aka JALH on irc.openprojects.net: #zealos and many more)
mark@zealos.org
mark@itsolve.co.uk

UL++++>$ G!>(GCM/GCS/GS/GM) dpu? s:-@ a16! C++++>$ P++++>+++++$ L+++>+++++$
!E---? W+++>$ N- !o? !w--- O? !M? !V? !PS !PE--@ PGP+? r++ !t---?@ !X---?
!R- b+ !tv b+ DI+ D+? G+++ e>+++++ !h++* r!-- y--

(www.geekcode.com)

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--- include/asm-i386/spinlock.h.old	Thu Oct 11 21:28:37 2001
+++ include/asm-i386/spinlock.h	Thu Oct 11 21:35:14 2001
@@ -58,8 +58,8 @@
 	"js 2f\n" \
 	".section .text.lock,\"ax\"\n" \
 	"2:\t" \
-	"cmpb $0,%0\n\t" \
 	"rep;nop\n\t" \
+	"cmpb $0,%0\n\t" \
 	"jle 2b\n\t" \
 	"jmp 1b\n" \
 	".previous"