Re: test13-pre5

[Linus Torvalds]

In article <[e-mail]>,
Matti Aarnio  <[e-mail]> 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