comment memory barriers

e7a5b3c5 · rsc · 0b75a8e8 · e7a5b3c5
--- a/spinlock.c
+++ b/spinlock.c
@@ -50,6 +50,10 @@ acquire(struct spinlock *lock)

  while(cmpxchg(0, 1, &lock->locked) == 1)
    ;
+
+  // Now that lock is acquired, make sure 
+  // we wait for all pending writes from other
+  // processors.
  cpuid(0, 0, 0, 0, 0);  // memory barrier
  
  // Record info about lock acquisition for debugging.
@@ -64,13 +68,16 @@ acquire(struct spinlock *lock)
 void
 release(struct spinlock *lock)
 {
-
  if(!holding(lock))
    panic("release");

  lock->pcs[0] = 0;
  lock->cpu = 0xffffffff;
+  
+  // Before unlocking the lock, make sure to flush
+  // any pending memory writes from this processor.
  cpuid(0, 0, 0, 0, 0);  // memory barrier
+
  lock->locked = 0;
  if(--cpus[cpu()].nlock == 0)
    sti();