CAS and spinlocks (was: Re: [f-cpu] another DATE report)

[Michael Riepe <michael@stud.uni-hannover.de>]

On Mon, Mar 18, 2002 at 03:49:07AM +0100, Christophe wrote:
[...]
> well, there is a problem with the spinlock version : what happens if a task
> switch occurs between the locking and the unlocking ? especially if other tasks
> will try to acquire the same spinlock, they all shall be blocked until the
> owner task may release the spinlock, which - not speaking about the invert
> priority problem of tasks - is in fact a real degradation. Such a thing cannot
> happen with CAS2 because the first to be serviced is the first to write, not
> the first to read - the other tasks would just need a retry - which takes
> basically less time to execute than waiting for owner task to unlock.

A spinlock protected piece of code should be as short as possible, and
not contain blocking functions at all. Spinlocks are just a way to make a
sequence of simple instructions atomic -- like the update operations in
the stack example, or a software implementation of CAS2 (which would be
too heavy as a machine instruction -- remember that it has six operands).

-- 
 Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
 "All I wanna do is have a little fun before I die"
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