diff options
| -rw-r--r-- | spinlock.c | 26 |
1 files changed, 14 insertions, 12 deletions
@@ -29,11 +29,14 @@ acquire(struct spinlock *lk) panic("acquire"); // The xchg is atomic. - // It also serializes, so that reads after acquire are not - // reordered before it. while(xchg(&lk->locked, 1) != 0) ; + // Tell the C compiler and the processor to not move loads or stores + // past this point, to ensure that the critical section's memory + // references happen after the lock is acquired. + __sync_synchronize(); + // Record info about lock acquisition for debugging. lk->cpu = cpu; getcallerpcs(&lk, lk->pcs); @@ -49,16 +52,15 @@ release(struct spinlock *lk) lk->pcs[0] = 0; lk->cpu = 0; - // The xchg serializes, so that reads before release are - // 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(&lk->locked, 0); + // Tell the C compiler and the processor to not move loads or stores + // past this point, to ensure that all the stores in the critical + // section are visible to other cores before the lock is released. + // Both the C compiler and the hardware may re-order loads and + // stores; __sync_synchronize() tells them both to not re-order. + __sync_synchronize(); + + // Release the lock. + lk->locked = 0; popcli(); } |