make grade 80/80 with comment

kernel/bio.c

@@ -98,20 +98,20 @@ bget(uint dev, uint blockno)
     }
     h = h->next;
   }
+  release(&hash_locks[bucket]); 
+  // release the unfound bucket, or there'll be deadlock
   acquire(&bcache.lock);
+  // after bcache.lock is aquired, only b->refcnt maybe out of our control
+  //    so, have to aquire hash_locks[victim_bucket] before test refcnt
   for(int i = 0; i < NBUF; ++ i){
     b = &bcache.buf[i];
+    int victim_bucket = b->blockno % NLOCK_BUCKETS;
+    acquire(&hash_locks[victim_bucket]);
     if(b->refcnt == 0) {
-      int victim_bucket = b->blockno % NLOCK_BUCKETS;
       b->dev = dev;
       b->blockno = blockno;
       b->valid = 0;
       b->refcnt = 1;
-      // first find the victim's hash
-      if (victim_bucket != bucket) 
-      {
-        // acquire(&hash_locks[victim_bucket]);
-      }
       // try to release hash entry from the old bucket
       h = hash_entry[victim_bucket];
       ph = 0;
@@ -124,19 +124,19 @@ bget(uint dev, uint blockno)
         ph = h;
         h = h->next;
       }
-      // add it to new hash entry
+      release(&hash_locks[victim_bucket]);
+      release(&bcache.lock);
+      // this should avoid deadlock and meanwhile guard the hashtable
+      // because after b is set, the only thing we need is hashtable rather than buf attributes
       h = &hash_tbls[i];
+      acquire(&hash_locks[bucket]);
       h->next = hash_entry[bucket];
       hash_entry[bucket] = h;
-
-      if (victim_bucket != bucket) {
-        // release(&hash_locks[victim_bucket]);
-      }
-      release(&bcache.lock);
       release(&hash_locks[bucket]);
       acquiresleep(&b->lock);
       return b;
     }
+    release(&hash_locks[victim_bucket]);
   }
 #else
   // Is the block already cached?
@@ -201,6 +201,11 @@ brelse(struct buf *b)
 
   releasesleep(&b->lock);
 
+// if you use bcache lock, test0 fails without doubt
+// the reason why this works is that, whenever b->refcnt is written or read
+//    its 'hash_locks[H(b->blockno)]' will always been locked
+// the reason why b->blockno is valid is that, only when refcnt==0 could this field be written
+//    but here it must be non-zero before lock aquired
   int bucket = b->blockno % NLOCK_BUCKETS;
   acquire(&hash_locks[bucket]);
   b->refcnt--;

time.txt

@@ -0,0 +1 @@
+9