fix an exit/exit deadlock -> one more locking protocol violation

increase timer rate from 1/second to 10/second

kernel/proc.c

@@ -294,17 +294,21 @@ reparent(struct proc *p, struct proc *parent) {
   int child_of_init = (p->parent == initproc);
 
   for(pp = proc; pp < &proc[NPROC]; pp++){
-    if (pp != p && pp != parent) {
+    // this code uses pp->parent without holding pp->lock.
+    // acquiring the lock first could cause a deadlock
+    // if pp or a child of pp were also in exit()
+    // and about to try to lock p.
+    if(pp->parent == p){
+      // pp->parent can't change between the check and the acquire()
+      // because only the parent changes it, and we're the parent.
       acquire(&pp->lock);
-      if(pp->parent == p){
-        pp->parent = initproc;
-        if(pp->state == ZOMBIE) {
-          if(!child_of_init)
-            acquire(&initproc->lock);
-          wakeup1(initproc);
-          if(!child_of_init)
-            release(&initproc->lock);
-        }
+      pp->parent = initproc;
+      if(pp->state == ZOMBIE) {
+        if(!child_of_init)
+          acquire(&initproc->lock);
+        wakeup1(initproc);
+        if(!child_of_init)
+          release(&initproc->lock);
       }
       release(&pp->lock);
     }

kernel/start.c

@@ -41,15 +41,16 @@ start()
   // which turns them into software interrupts for
   // devintr() in trap.c.
   int id = r_mhartid();
-  // ask the CLINT for a timer interrupt 10,000 cycles from now.
-  *(uint64*)CLINT_MTIMECMP(id) = *(uint64*)CLINT_MTIME + 10000;
+  // ask the CLINT for a timer interrupt.
+  int interval = 1000000; // cycles; about 1/10th second in qemu.
+  *(uint64*)CLINT_MTIMECMP(id) = *(uint64*)CLINT_MTIME + interval;
   // prepare information in scratch[] for machinevec.
   // scratch[0..3] : space for machinevec to save registers.
   // scratch[4] : address of CLINT MTIMECMP register.
   // scratch[5] : desired interval (in cycles) between timer interrupts.
   uint64 *scratch = &mscratch0[32 * id];
   scratch[4] = CLINT_MTIMECMP(id);
-  scratch[5] = 10000000;
+  scratch[5] = interval;
   w_mscratch((uint64)scratch);
   // set the machine-mode trap handler.
   w_mtvec((uint64)machinevec);
@@ -61,6 +62,6 @@ start()
   // keep each CPU's hartid in its tp register, for cpuid().
   w_tp(id);
 
-  // call main() in supervisor mode.
+  // switch to supervisor mode and jump to main().
   asm volatile("mret");
 }

user/usertests.c

@@ -448,7 +448,7 @@ reparent(void)
   
   printf(1, "reparent test\n");
 
-  for(int i = 0; i < 100; i++){
+  for(int i = 0; i < 200; i++){
     int pid = fork();
     if(pid < 0){
       printf(1, "fork failed\n");
@@ -571,10 +571,10 @@ forkforkfork(void)
     exit();
   }
 
-  sleep(2);
+  sleep(20); // two seconds
   close(open("stopforking", O_CREATE|O_RDWR));
   wait();
-  sleep(1);
+  sleep(10); // one second
 
   printf(1, "forkforkfork ok\n");
 }