Simple Software Lock

lock: ld R1,loc -- get fresh value

cmp loc, #0 -- test if it changed

bnz R1,lock -- spin if loc ~ free?

st loc, #1 -- its free, set it

ret

and

unlock: st loc,#0 -- clear setting

ret

The code seems simple enough, but consider various interleavings

Previous slide

Next slide

Back to first slide

View graphic version