Re: PATCH: Fix ll/sc for mips (take 3)

[Justin Carlson <justinca@ri.cmu.edu>]

On Mon, 2002-02-04 at 23:47, Daniel Jacobowitz wrote:
> 
> Won't this cause some gratuitous thrashing if someone else is trying to
> get the spinlock at the same time?
> 

Actually, if you look at the required semantics of ll, no.  A ll by
itself can never cause a sc from another cpu to fail.  It's part of the
semantic definition to avoid potential livelock cases, e.g.

A does ll
B does ll, foiling A's lock attempt
A does sc, which fails
A does ll, foiling B's lock attempt
B does sc, which fails
B does ll, foiling A's lock attempt
...

Instead, this case becomes:
A does ll
B does ll
A does sc, which succeeds, even though B has done a ll
B does sc which fails
A does critical section work
B spins on ll until A releases the lock


-Justin