diff options
| -rw-r--r-- | spinlock.c | 12 |
1 files changed, 8 insertions, 4 deletions
@@ -54,10 +54,14 @@ release(struct spinlock *lock) lock->cpu = 0xffffffff; // The xchg serializes, so that reads before release are - // not reordered after it. (This reordering would be allowed - // by the Intel manuals, but does not happen on current - // Intel processors. The xchg being asm volatile also keeps - // gcc from delaying the above assignments.) + // not reordered after it. The 1996 PentiumPro manual (Volume 3, + // 7.2) says reads can be carried out speculatively and in + // any order, which implies we need to serialize here. + // But the 2007 Intel 64 Architecture Memory Ordering White + // Paper says that Intel 64 and IA-32 will not move a load + // after a store. So lock->locked = 0 would work here. + // The xchg being asm volatile ensures gcc emits it after + // the above assignments (and after the critical section). xchg(&lock->locked, 0); popcli(); |