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From
SubjectRe: test13-pre5
Date
In article <20001231200741.F28963@mea-ext.zmailer.org>,
Matti Aarnio <matti.aarnio@zmailer.org> wrote:
>
> Actually nothing SMP specific in that problem sphere.
> Alpha has load-locked/store-conditional pair for
> this type of memory accesses to automatically detect,
> and (conditionally) restart the operation - to form
> classical ``locked-read-modify-write'' operations.

Sure, we could make the older alphas use ldl_l stl_c for byte accesses,
but if you thought byte accesses on those machines were kind-of slow
before, just WAIT until that happens.

Old alpha machines (the same ones that would need this code) were
HORRIBLE at ldl_l<->stl_c: they go out all the way to the bus to set the
lock. So suddenly your every byte access ends up being a few hundred
cycles!

So ldl_l/stc_l is not the answer. It would work, but it would be so
slow that you'd be a lot better off not doing it.

I think they fixed ldl/stc later on (so that it only sets a bit locally
that gets cleared by the cache coherency protocol), but as later alphas
have the byte accesses anyway that doesn't matter here. The faster
ldl/stc makes for much faster spinlocks on newer alphas, though.

Linus
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