Is Parallel Programming Hard, And, If So, What Can You Do About It?

12.2. MEMORY BARRIERS 133CPU 1000 111000 111 A=1000 111000 111 B=2000 111wwwwwwwwwwwwwwww0000 11110000 1111A−>00000 1111B−>90000 11110000 11110000 1111B−>20000 11110000 1111A−>10000 11110000 1111A−>10000 1111rrrrrrrrrrrrrrrrrCPU 21st2ndThe CPU being busy doing adivision speculates on theLOAD of AFigure 12.13: Read Barrier Supplied, Take TwoOnce the divisions are completethe CPU can then perform theLOAD with immediate effect0000 11110000 1111 B−>20000 11110000 1111 A−>00000 11110000 11110000 11110000 1111DIVIDEDIVIDECPU 2Figure 12.14: Speculative Load12.2.12 Memory-Barrier Examples12.2.12.1 Locking ExamplesLOCK Followed by UNLOCK: A LOCK followedbyanUNLOCKmaynotbeassumedtobefullmemory barrier because it is possible for an accessprecedingtheLOCKtohappenaftertheLOCK,andan access following the UNLOCK to happen beforethe UNLOCK, and the two accesses can themselvesthen cross. For example, the following:1 *A = a;2 LOCK3 UNLOCK4 *B = b;might well execute in the following order:2 LOCK4 *B = b;1 *A = a;3 UNLOCKAgain, always remember that both LOCK andUNLOCK are permitted to let preceding operations“bleed in” to the critical section.Quick Quiz 12.11: What sequence of LOCK-UNLOCK operations would act as a full memorybarrier?Quick Quiz 12.12: What (if any) CPUs havememory-barrier instructions from which these semipermiablelocking primitives might be constructed?LOCK-Based Critical Sections: Although aLOCK-UNLOCK pair does not act as a full memorybarrier, these operations do affect memory ordering.Consider the following code:1 *A = a;2 *B = b;3 LOCK4 *C = c;5 *D = d;6 UNLOCK7 *E = e;8 *F = f;This could legimately execute in the following order,where pairs of operations on the same line indicatethat the CPU executed those operations concurrently: