ph ok,though nothing interesting

answers-thread.txt

@@ -0,0 +1,12 @@
+Q: Why are there missing keys with 2 threads, but not with 1 thread? Identify a sequence of events with 2 threads that can lead to a key being missing. 
+A:  Suppose K1 % NBUCKET === K2 % NBUCKET = i0
+    T1 put(K1);
+    T2 put(K2);
+    T1 not found
+    T2 not found
+    T1 insert(K1, V1, &table[i0], table[i0])
+    T2 insert(K2, V2, &table[i0], table[i0])
+    T1 malloc e1, e1->key = K1, e1->value = V1, e1->next = table[i0]
+    T2 malloc e2, e2->key = K2, e2->value = V2, e2->next = table[i0]
+    T1 *(&table[i0]) = e1
+    T2 *(&table[i0]) = e2 <---- K1 lost

notxv6/ph.c

@@ -16,7 +16,7 @@ struct entry {
 struct entry *table[NBUCKET];
 int keys[NKEYS];
 int nthread = 1;
-
+pthread_mutex_t lock[NBUCKET];
 
 double
 now()
@@ -43,6 +43,7 @@ void put(int key, int value)
 
   // is the key already present?
   struct entry *e = 0;
+  pthread_mutex_lock(&lock[i]);
   for (e = table[i]; e != 0; e = e->next) {
     if (e->key == key)
       break;
@@ -54,6 +55,7 @@ void put(int key, int value)
     // the new is new.
     insert(key, value, &table[i], table[i]);
   }
+  pthread_mutex_unlock(&lock[i]);
 
 }
 
@@ -64,10 +66,11 @@ get(int key)
 
 
   struct entry *e = 0;
+  pthread_mutex_lock(&lock[i]);
   for (e = table[i]; e != 0; e = e->next) {
     if (e->key == key) break;
   }
-
+  pthread_mutex_unlock(&lock[i]);
   return e;
 }
 
@@ -117,7 +120,9 @@ main(int argc, char *argv[])
   for (int i = 0; i < NKEYS; i++) {
     keys[i] = random();
   }
-
+  for (int i = 0; i < NBUCKET; i ++){
+    pthread_mutex_init(&lock[i], NULL);
+  }
   //
   // first the puts
   //