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Re: Build failure for R3000 DECstation

To: "Maciej W. Rozycki" <>
Subject: Re: Build failure for R3000 DECstation
From: Jun Sun <>
Date: Wed, 15 Nov 2000 12:21:54 -0800
Cc: Harald Koerfgen <>,, Ralf Baechle <>
References: <>
"Maciej W. Rozycki" wrote:
> On Wed, 15 Nov 2000, Harald Koerfgen wrote:
> > On 15-Nov-00 Jun Sun wrote:
> > [R3000 UP userland spinlocks]
> > > In fact, I don't think you can perform automic operation ONLY based on
> > > the knowledge whether a context switch has happened during a specified
> > > period.  (It should be interesting to see if we can actually "prove"
> > > it.)
> >
> > I doubt this as well, although I'd love to be proven wrong.
>  Well, on UP the only events that can break atomicity are exceptions (here
> I treat interrupts as exceptions as well) and DMA accesses.  I don't think
> we do DMA to user space, so this should not be a problem.  So if we can
> detect an exception occured we may assume an operation failed and retry.
> It's not a problem for an exception handler to clobber k0 or k1 upon exit.

I gave more thoughts on this.  While your argument sounds plausible, the
devil is in "retry" - without a lower-level atomic operation, you cannot
"restore" the initial condition and conduct a re-try.

Come up with a pseudo code to show I am wrong.

>  Unfortunately we cannot use this implementation in the userland or we
> risk problems when running on SMP systems -- an ISA-I user binary might
> very well be run on an ISA-II (or higher) SMP system.  But we can use it
> in the kernel, for sysmips() and everything else.  All we have to be
> careful about is not to allow DMA accesses to spinlocks.  I don't think
> this is a problem in reality.

Let us solve UP first. :-)


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