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To: Chris Csanady <ccsanady@friley216.res.iastate.edu>
cc: freebsd-smp@freebsd.org
Subject: Re: spin locks... 
In-reply-to: Your message of "Tue, 03 Dec 1996 00:54:40 CST."
             <199612030654.AAA12221@friley216.res.iastate.edu> 
Date: Tue, 03 Dec 1996 08:42:04 +0100
Message-ID: <3915.849598924@critter.tfs.com>
From: Poul-Henning Kamp <phk@critter.tfs.com>
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In message <199612030654.AAA12221@friley216.res.iastate.edu>, Chris Csanady wri
tes:
>Its something we could use, so I thought I'd give it a shot.  I thought it
>would make a nice first assembly project, right?  Yeah.. sure.  I have never
>been so frustrated with something so obviously simple.
>
>When I try to link it into a test program, it segfaults.  The only way I
>could obtain predictable behavior was to take out the cmpxchg and lock.
>
>Could someone please point out my error here?
>
>.text
>        .align  2,0x90
>.globl _get_spinlock
>_get_spinlock:
>        pushl   %eax
>        pushl   %ecx
>        pushl   %edx
>        movl    _smp_active, %eax
>        cmpl    $0, %eax
+	 je	4
->        jne     1f
->        jmp     4
>1:      movl    4(%esp), %edx
>        movl    $1, %ecx
>2:      lock
>        cmpxchg %ecx, (%edx)
I can't remember, that lock may be implied in the cmpxchg instruction...
Check out the smplock() I wrote and see what I do.

->        je      3f
->        jmp     4
+	 jne	 4
>3:      cmpl    %ecx, (%edx)
+	 jne	2b

>        je      3b
I'm sure not, but where do you want to go then ?

->        jmp     2b
>4:      popl    %edx
>        popl    %ecx
>        popl    %eax
>        ret
>
>--Chris Csanady
>
>
>

--
Poul-Henning Kamp           | phk@FreeBSD.ORG       FreeBSD Core-team.
http://www.freebsd.org/~phk | phk@login.dknet.dk    Private mailbox.
whois: [PHK]                | phk@tfs.com           TRW Financial Systems, Inc.
Power and ignorance is a disgusting cocktail.