another test, to help with locking exercises

kernel/kalloc.c

@@ -55,8 +55,9 @@ kfree(void *pa)
   // Fill with junk to catch dangling refs.
   memset(pa, 1, PGSIZE);
 
-  acquire(&kmem.lock);
   r = (struct run*)pa;
+
+  acquire(&kmem.lock);
   r->next = kmem.freelist;
   kmem.freelist = r;
   release(&kmem.lock);
@@ -75,6 +76,7 @@ kalloc(void)
   if(r)
     kmem.freelist = r->next;
   release(&kmem.lock);
+
   if(r)
     memset((char*)r, 5, PGSIZE); // fill with junk
   return (void*)r;

kernel/spinlock.c

@@ -25,7 +25,7 @@ acquire(struct spinlock *lk)
   if(holding(lk))
     panic("acquire");
 
-  // On RISC-V, this turns into an atomic swap:
+  // On RISC-V, sync_lock_test_and_set turns into an atomic swap:
   //   a5 = 1
   //   s1 = &lk->locked
   //   amoswap.w.aq a5, a5, (s1)
@@ -57,9 +57,10 @@ release(struct spinlock *lk)
   __sync_synchronize();
 
   // Release the lock, equivalent to lk->locked = 0.
-  // This code can't use a C assignment, since it might
-  // not be atomic.
-  // On RISC-V, this turns into an atomic swap:
+  // This code doesn't use a C assignment, since the C standard
+  // implies that an assignment might be implemented with
+  // multiple store instructions.
+  // On RISC-V, sync_lock_release turns into an atomic swap:
   //   s1 = &lk->locked
   //   amoswap.w zero, zero, (s1)
   __sync_lock_release(&lk->locked);

user/usertests.c

@@ -506,6 +506,44 @@ twochildren(void)
   printf(1, "twochildren ok\n");
 }
 
+// concurrent forks to try to expose locking bugs.
+void
+forkfork(void)
+{
+  int ppid = getpid();
+  
+  printf(1, "forkfork test\n");
+
+  for(int i = 0; i < 2; i++){
+    int pid = fork();
+    if(pid < 0){
+      printf(1, "fork failed");
+      exit();
+    }
+    if(pid == 0){
+      for(int j = 0; j < 200; j++){
+        int pid1 = fork();
+        if(pid1 < 0){
+          printf(1, "fork failed\n");
+          kill(ppid);
+          exit();
+        }
+        if(pid1 == 0){
+          exit();
+        }
+        wait();
+      }
+      exit();
+    }
+  }
+
+  for(int i = 0; i < 2; i++){
+    wait();
+  }
+
+  printf(1, "forkfork ok\n");
+}
+
 void
 forkforkfork(void)
 {
@@ -1858,6 +1896,7 @@ main(int argc, char *argv[])
 
   reparent();
   twochildren();
+  forkfork();
   forkforkfork();
   
   argptest();