Simple Lock with LL-SC

lock: ll reg1, location /* LL location to reg1 */

sc location, reg2 /* SC reg2 into location*/
beqz reg2, lock /* if failed, start again */
ret
unlock: st location, #0 /* write 0 to location */
ret

• Can do more fancy atomic ops by changing what’s between LL & SC

  • But keep it small so SC likely to succeed
  • Don’t include instructions that would need to be undone (e.g. stores)

• SC can fail (without putting transaction on bus) if:

  • Detects intervening write even before trying to get bus
  • Tries to get bus but another processor’s SC gets bus first

• LL, SC are not lock, unlock respectively

  • Only guarantee no conflicting write to lock variable between them
  • But can use directly to implement simple operations on shared variables

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