Date: Tue, 29 Jul 1997 22:18:43 -0700 From: "Michael L. VanLoon -- HeadCandy.com" <michaelv@MindBender.serv.net> To: dg@root.com Cc: Christopher Ulbright <ulbright@cs.washington.edu>, smp@freebsd.org Subject: Re: Input on spl changes Message-ID: <199707300518.WAA06147@MindBender.serv.net> In-Reply-To: Your message of Tue, 29 Jul 97 17:58:27 -0700. <199707300058.RAA10698@implode.root.com>
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>>My initial reaction was to make per-processor cpl variables but this in
>>and of itself will not solve the problem. This is because when thread-1
>>on cpu0 sets cpl to disable certain INTs, thread-2 on cpu1 could still
>>enter some code section that thread-1(cpu0) assumes it has exclusive
>>access to. Steve P. informed me that a per-cpu cpl wasn't a good idea.
>>
>>New idea: Have per-cpu cpl variables and restrict access to any FreeBSD
>>code to one cpu at a time. Each time a thread enters FBSD code, it will
>>have restored it's appropriate cpl value. If it needs to enter code
>>requiring the masking of INTs, the cpu-specific cpl will be set
>>accordingly.
>>
>>Restricted access could be achieved through use of a lock similar to
>>Steve's mp_lock.
>>
>>Does anyone see the faults in this idea? Are there other problems with
>>per-cpu cpls that I need to consider?
>>
>>Thanks for any thoughts/ideas/advice.
> Hmmm. per-cpu cpl doesn't make any sense to me - at least not in the
>current paradigm. We're using cpl as a global lock on interrupt reentrancy
>and this is not a cpu-specific thing. I think the current SMP code blocks
>having concurrent CPUs in interrupt code, but this restriction shouldn't
>be necessary as long as cpl functions as a global lock.
It sounds like what you're trying to do would be better achieved with
"standard" critical sections, or something similar. It would only
allow one thread into a specific area of code at a time, but would not
block the other processor if it wasn't trying to run in that piece of
code. This, of course, would have no affect on interrupt levels.
-----------------------------------------------------------------------------
Michael L. VanLoon michaelv@MindBender.serv.net
--< Free your mind and your machine -- NetBSD free un*x >--
NetBSD working ports: 386+PC, Mac 68k, Amiga, Atari 68k, HP300, Sun3,
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NetBSD ports in progress: PICA, others...
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