Date: Tue, 27 Jun 2000 10:42:20 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Chuck Paterson <cp@bsdi.com> Cc: David Greenman <dg@root.com>, freebsd-smp@freebsd.org Subject: Re: Stepping on Toes Message-ID: <200006271742.KAA35851@apollo.backplane.com> References: <200006261650.KAA17801@berserker.bsdi.com>
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:} Even with interrupt threads we have the GiantMutex issue... the same
:} issue that we have with our current MP implementation. We cannot remove
:} SPL's until we remove the GiantMutex, and we cannot remove GiantMutex
:} without major modifications to just about every single source file in sys/
:
:In general this isn't true. If you get to the point where
:
:1) All entrance to unsafe code is proteced by Giant.
:2) Tsleep and friend if any release Giant when they
: a process is suspended and re-acquire it on exit
:3) Interrupts have a context to run in.
:4) You have one or more scheduling locks.
:
:Then you can just turn spls into a nop. There is lots of hand waving
:in regards to details at this point. BSD/OS SMPng is the existance
:proof.
:
:
:It seems like one of the major problems in retaining spls during
:the change over period is that they don't much useful, and effectively
:push everything under Giant.
:
: Grabbing a spl will only block interrupts, it will not give
: any protection against an interrupt thread which has already
: started.
:
:This means that any device which might be blocked by splbio() can
:not be brought out from under the Giant lock until all instances
:of splbio have been removed.
:
:Chuck
Yes, I see it. I agree. You don't even need to hold a scheduling
lock... all you need to hold is Giant.
#1 - done
#2 - done
#3 - (Greg)
#4 - not required
Right this moment the requirement is that only someone holding Giant
is allowed to mess with spl*()'s (the cpl variable can only be messed
with by people holding Giant).
At this moment, without interrupt threads, interrupts can share Giant
with the curproc they interrupted. This is how our existing MP stuff
worked already.
When Greg moves interrupts to their own threads, and obtains Giant to
run those interrupts, no more sharing will occur and just the fact
that the interrupt is holding Giant guarentees that nobody else will
be messing with SPLs, thus the SPLs can be removed entirely.
-Matt
Matthew Dillon
<dillon@backplane.com>
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