no more recursive locks

wakeup1() assumes you hold proc_table_lock
sleep(chan, lock) provides atomic sleep-and-release to wait for condition
ugly code in swtch/scheduler to implement new sleep
fix lots of bugs in pipes, wait, and exit
fix bugs if timer interrupt goes off in schedule()
console locks per line, not per byte

Notes

@@ -126,3 +126,26 @@ nasty hack to allow locks before first process,
 
 race between release and sleep in sys_wait()
 race between sys_exit waking up parent and setting state=ZOMBIE
+race in pipe code when full/empty
+
+lock order
+  per-pipe lock
+  proc_table_lock fd_table_lock kalloc_lock
+  console_lock
+
+condition variable + mutex that protects it
+  proc * (for wait()), proc_table_lock
+  pipe structure, pipe lock
+
+systematic way to test sleep races?
+  print something at the start of sleep?
+
+do you have to be holding the mutex in order to call wakeup()?
+
+should lock around printf, not putc
+
+device interrupts don't clear FL_IF
+  so a recursive timer interrupt is possible
+
+the sleep/swtch/schedule code that holds over a lock is ugly
+

console.c

@@ -4,7 +4,8 @@
 #include "spinlock.h"
 
 struct spinlock console_lock;
-int use_printf_lock = 0;
+int paniced = 0;
+int use_console_lock = 0;
 
 /*
  * copy console output to parallel port, which you can tell
@@ -23,15 +24,18 @@ lpt_putc(int c)
 	outb(0x378+2, 0x08);
 }
 
-void
+static void
-cons_putc(int c)
+real_cons_putc(int c)
 {
   int crtport = 0x3d4; // io port of CGA
   unsigned short *crt = (unsigned short *) 0xB8000; // base of CGA memory
   int ind;
 
-  if(use_printf_lock)
+  if(paniced){
-    acquire(&console_lock);
+    cli();
+    while(1)
+      ;
+  }
 
   lpt_putc(c);
 
@@ -63,8 +67,15 @@ cons_putc(int c)
   outb(crtport + 1, ind >> 8);
   outb(crtport, 15);
   outb(crtport + 1, ind);
+}
 
-  if(use_printf_lock)
+void
+cons_putc(int c)
+{
+  if(use_console_lock)
+    acquire(&console_lock);
+  real_cons_putc(c);
+  if(use_console_lock)
     release(&console_lock);
 }
 
@@ -91,7 +102,7 @@ printint(int xx, int base, int sgn)
 
   while(i > 0){
     i -= 1;
-    cons_putc(buf[i]);
+    real_cons_putc(buf[i]);
   }
 }
 
@@ -104,13 +115,16 @@ cprintf(char *fmt, ...)
   int i, state = 0, c;
   unsigned int *ap = (unsigned int *) &fmt + 1;
 
+  if(use_console_lock)
+    acquire(&console_lock);
+
   for(i = 0; fmt[i]; i++){
     c = fmt[i] & 0xff;
     if(state == 0){
       if(c == '%'){
         state = '%';
       } else {
-        cons_putc(c);
+        real_cons_putc(c);
       }
     } else if(state == '%'){
       if(c == 'd'){
@@ -120,20 +134,25 @@ cprintf(char *fmt, ...)
         printint(*ap, 16, 0);
         ap++;
       } else if(c == '%'){
-        cons_putc(c);
+        real_cons_putc(c);
       }
       state = 0;
     }
   }
+
+  if(use_console_lock)
+    release(&console_lock);
 }
 
 void
 panic(char *s)
 {
-  use_printf_lock = 0;
+  __asm __volatile("cli");
+  use_console_lock = 0;
   cprintf("panic: ");
   cprintf(s, 0);
   cprintf("\n", 0);
+  paniced = 1; // freeze other CPU
   while(1)
     ;
 }

defs.h

@@ -14,7 +14,8 @@ struct jmpbuf;
 void setupsegs(struct proc *);
 struct proc * newproc(void);
 void swtch(int);
-void sleep(void *);
+struct spinlock;
+void sleep(void *, struct spinlock *);
 void wakeup(void *);
 void scheduler(void);
 void proc_exit(void);
@@ -65,6 +66,8 @@ int cpu(void);
 struct spinlock;
 void acquire(struct spinlock * lock);
 void release(struct spinlock * lock);
+void acquire1(struct spinlock * lock, struct proc *);
+void release1(struct spinlock * lock, struct proc *);
 
 // main.c
 void load_icode(struct proc *p, uint8_t *binary, unsigned size);

ide.c

@@ -112,7 +112,7 @@ ide_start_read(uint32_t secno, void *dst, unsigned nsecs)
     panic("ide_start_read: nsecs too large");
 
   while ((head + 1) % NREQUEST == tail)
-    sleep (&disk_channel);
+    sleep (&disk_channel, 0);
 
   r = &request[head];
   r->secno = secno;

main.c

@@ -16,7 +16,7 @@ extern char _binary_user1_start[], _binary_user1_size[];
 extern char _binary_usertests_start[], _binary_usertests_size[];
 extern char _binary_userfs_start[], _binary_userfs_size[];
 
-extern use_printf_lock;
+extern int use_console_lock;
 
 int
 main()
@@ -40,7 +40,7 @@ main()
 
   mp_init(); // collect info about this machine
 
-  use_printf_lock = 1;
+  use_console_lock = 1;
 
   cpus[cpu()].clis = 1; // cpu starts as if we had called cli()
 

pipe.c

@@ -81,16 +81,17 @@ pipe_write(struct pipe *p, char *addr, int n)
 
   for(i = 0; i < n; i++){
     while(((p->writep + 1) % PIPESIZE) == p->readp){
-      if(p->readopen == 0)
+      if(p->readopen == 0){
+        release(&p->lock);
         return -1;
-      release(&p->lock);
+      }
       wakeup(&p->readp);
-      sleep(&p->writep);
+      sleep(&p->writep, &p->lock);
-      acquire(&p->lock);
     }
     p->data[p->writep] = addr[i];
     p->writep = (p->writep + 1) % PIPESIZE;
   }
+
   release(&p->lock);
   wakeup(&p->readp);
   return i;
@@ -101,19 +102,23 @@ pipe_read(struct pipe *p, char *addr, int n)
 {
   int i;
 
+  acquire(&p->lock);
+
   while(p->readp == p->writep){
-    if(p->writeopen == 0)
+    if(p->writeopen == 0){
+      release(&p->lock);
       return 0;
-    sleep(&p->readp);
+    }
+    sleep(&p->readp, &p->lock);
   }
 
-  acquire(&p->lock);
   for(i = 0; i < n; i++){
     if(p->readp == p->writep)
       break;
     addr[i] = p->data[p->readp];
     p->readp = (p->readp + 1) % PIPESIZE;
   }
+
   release(&p->lock);
   wakeup(&p->writep);
   return i;

proc.c

@@ -95,7 +95,6 @@ newproc()
   np->tf = (struct Trapframe *) (np->kstack + KSTACKSIZE - sizeof(struct Trapframe));
   *(np->tf) = *(op->tf);
   np->tf->tf_regs.reg_eax = 0; // so fork() returns 0 in child
-  cprintf("newproc pid=%d return to %x:%x tf-%p\n", np->pid, np->tf->tf_cs, np->tf->tf_eip, np->tf);
 
   // set up new jmpbuf to start executing at trapret with esp pointing at tf
   memset(&np->jmpbuf, 0, sizeof np->jmpbuf);
@@ -109,8 +108,6 @@ newproc()
       fd_reference(np->fds[fd]);
   }
 
-  cprintf("newproc %x\n", np);
-
   return np;
 }
 
@@ -126,18 +123,27 @@ scheduler(void)
   setjmp(&cpus[cpu()].jmpbuf);
 
   op = curproc[cpu()];
+
+  if(op == 0 || op->mtx != &proc_table_lock)
+    acquire1(&proc_table_lock, op);
+
   if(op){
     if(op->newstate <= 0 || op->newstate > ZOMBIE)
       panic("scheduler");
     op->state = op->newstate;
     op->newstate = -1;
+    if(op->mtx){
+      struct spinlock *mtx = op->mtx;
+      op->mtx = 0;
+      if(mtx != &proc_table_lock)
+        release1(mtx, op);
+    }
   }
 
   // find a runnable process and switch to it
   curproc[cpu()] = 0;
   np = cpus[cpu()].lastproc + 1;
   while(1){
-    acquire(&proc_table_lock);
     for(i = 0; i < NPROC; i++){
       if(np >= &proc[NPROC])
         np = &proc[0];
@@ -148,11 +154,13 @@ scheduler(void)
 
     if(i < NPROC){
       np->state = RUNNING;
-      release(&proc_table_lock);
+      release1(&proc_table_lock, op);
       break;
     }
     
-    release(&proc_table_lock);
+    release1(&proc_table_lock, op);
+    op = 0;
+    acquire(&proc_table_lock);
     np = &proc[0];
   }
 
@@ -180,36 +188,56 @@ void
 swtch(int newstate)
 {
   struct proc *p = curproc[cpu()];
+
   if(p == 0)
     panic("swtch no proc");
-  if(p->locks != 0)
+  if(p->mtx == 0 && p->locks != 0)
     panic("swtch w/ locks");
+  if(p->mtx && p->locks != 1)
+    panic("swtch w/ locks 1");
+  if(p->mtx && p->mtx->locked == 0)
+    panic("switch w/ lock but not held");
+  if(p->locks && (read_eflags() & FL_IF))
+    panic("swtch w/ lock but FL_IF");
+
   p->newstate = newstate; // basically an argument to scheduler()
   if(setjmp(&p->jmpbuf) == 0)
     longjmp(&cpus[cpu()].jmpbuf);
 }
 
 void
-sleep(void *chan)
+sleep(void *chan, struct spinlock *mtx)
 {
   struct proc *p = curproc[cpu()];
+
   if(p == 0)
     panic("sleep");
+
   p->chan = chan;
+  p->mtx = mtx; // scheduler will release it
+
   swtch(WAITING);
+  
+  if(mtx)
+    acquire(mtx);
+  p->chan = 0;
+}
+
+void
+wakeup1(void *chan)
+{
+  struct proc *p;
+
+  for(p = proc; p < &proc[NPROC]; p++)
+    if(p->state == WAITING && p->chan == chan)
+      p->state = RUNNABLE;
 }
 
 void
 wakeup(void *chan)
 {
-  struct proc *p;
-
   acquire(&proc_table_lock);
-  for(p = proc; p < &proc[NPROC]; p++){
+  wakeup1(chan);
-    if(p->state == WAITING && p->chan == chan){
-      p->state = RUNNABLE;
-    }
-  }
   release(&proc_table_lock);
 }
 
@@ -229,8 +257,6 @@ proc_exit()
   struct proc *cp = curproc[cpu()];
   int fd;
 
-  cprintf("exit %x pid %d ppid %d\n", cp, cp->pid, cp->ppid);
-
   for(fd = 0; fd < NOFILE; fd++){
     if(cp->fds[fd]){
       fd_close(cp->fds[fd]);
@@ -243,32 +269,35 @@ proc_exit()
   // wake up parent
   for(p = proc; p < &proc[NPROC]; p++)
     if(p->pid == cp->ppid)
-      wakeup(p);
+      wakeup1(p);
 
   // abandon children
   for(p = proc; p < &proc[NPROC]; p++)
     if(p->ppid == cp->pid)
       p->pid = 1;
   
-  release(&proc_table_lock);
+  cp->mtx = &proc_table_lock;
-
-  // switch into scheduler
   swtch(ZOMBIE);
+  panic("a zombie revived");
 }
 
 // disable interrupts
 void
 cli(void)
 {
-  cpus[cpu()].clis += 1;
+  if(cpus[cpu()].clis == 0)
-  if(cpus[cpu()].clis == 1)
     __asm __volatile("cli");
+  cpus[cpu()].clis += 1;
+  if((read_eflags() & FL_IF) != 0)
+    panic("cli but enabled");
 }
 
 // enable interrupts
 void
 sti(void)
 {
+  if((read_eflags() & FL_IF) != 0)
+    panic("sti but enabled");
   if(cpus[cpu()].clis < 1)
     panic("sti");
   cpus[cpu()].clis -= 1;

proc.h

@@ -41,6 +41,7 @@ struct proc{
   char *kstack; // kernel stack, separate from mem so it doesn't move
   enum proc_state state;
   enum proc_state newstate; // desired state after swtch()
+  struct spinlock *mtx; // mutex for condition variable
   int pid;
   int ppid;
   void *chan; // sleep

spinlock.c

@@ -8,36 +8,20 @@
 
 #define DEBUG 0
 
-extern use_printf_lock;
+extern int use_console_lock;
 
 int getcallerpc(void *v) {
   return ((int*)v)[-1];
 }
 
 void
-acquire(struct spinlock * lock)
+acquire1(struct spinlock * lock, struct proc *cp)
 {
-  struct proc *cp = curproc[cpu()];
-  unsigned who;
-
-  if(cp)
-    who = (unsigned) cp;
-  else
-    who = cpu() + 1;
-
   if(DEBUG) cprintf("cpu%d: acquiring at %x\n", cpu(), getcallerpc(&lock));
 
-  if (lock->who == who && lock->locked){
+  cli();
-    lock->count += 1;
+  while ( cmpxchg(0, 1, &lock->locked) == 1 ) { ; }
-  } else { 
+  lock->locker_pc = getcallerpc(&lock);
-    cli();
-    // if we get the lock, eax will be zero
-    // if we don't get the lock, eax will be one
-    while ( cmpxchg(0, 1, &lock->locked) == 1 ) { ; }
-    lock->locker_pc = getcallerpc(&lock);
-    lock->count = 1;
-    lock->who = who;
-  }
 
   if(cp)
     cp->locks += 1;
@@ -46,27 +30,29 @@ acquire(struct spinlock * lock)
 }
 
 void
-release(struct spinlock * lock)
+release1(struct spinlock * lock, struct proc *cp)
 {
-  struct proc *cp = curproc[cpu()];
-  unsigned who;
-
-  if(cp)
-    who = (unsigned) cp;
-  else
-    who = cpu() + 1;
 
   if(DEBUG) cprintf ("cpu%d: releasing at %x\n", cpu(), getcallerpc(&lock));
 
-  if(lock->who != who || lock->count < 1 || lock->locked != 1)
+  if(lock->locked != 1)
     panic("release");
 
-  lock->count -= 1;
   if(cp)
     cp->locks -= 1;
-  if(lock->count < 1){
+
-    lock->who = 0;
+  cmpxchg(1, 0, &lock->locked);
-    cmpxchg(1, 0, &lock->locked);
+  sti();
-    sti();
+}
-  }
+
+void
+acquire(struct spinlock *lock)
+{
+  acquire1(lock, curproc[cpu()]);
+}
+
+void
+release(struct spinlock *lock)
+{
+  release1(lock, curproc[cpu()]);
 }

spinlock.h

@@ -1,6 +1,4 @@
 struct spinlock {
   unsigned int locked;
-  unsigned who;
-  int count;
   unsigned locker_pc;
 };

syscall.c

@@ -152,8 +152,12 @@ sys_fork()
   struct proc *np;
 
   np = newproc();
-  np->state = RUNNABLE;
+  if(np){
-  return np->pid;
+    np->state = RUNNABLE;
+    return np->pid;
+  } else {
+    return -1;
+  }
 }
 
 int
@@ -170,11 +174,10 @@ sys_wait()
   struct proc *cp = curproc[cpu()];
   int any, pid;
 
-  cprintf("waid pid %d ppid %d\n", cp->pid, cp->ppid);
+  acquire(&proc_table_lock);
 
   while(1){
     any = 0;
-    acquire(&proc_table_lock);
     for(p = proc; p < &proc[NPROC]; p++){
       if(p->state == ZOMBIE && p->ppid == cp->pid){
         kfree(p->mem, p->sz);
@@ -182,18 +185,16 @@ sys_wait()
         pid = p->pid;
         p->state = UNUSED;
         release(&proc_table_lock);
-        cprintf("%x collected %x\n", cp, p);
         return pid;
       }
       if(p->state != UNUSED && p->ppid == cp->pid)
         any = 1;
     }
-    release(&proc_table_lock);
     if(any == 0){
-      cprintf("%x nothing to wait for\n", cp);
+      release(&proc_table_lock);
       return -1;
     }
-    sleep(cp);
+    sleep(cp, &proc_table_lock);
   }
 }
 
@@ -220,7 +221,7 @@ sys_block(void)
       panic("couldn't start read\n");
     }
     cprintf("call sleep\n");
-    sleep (c);
+    sleep (c, 0);
     if (ide_finish_read(c)) {
       panic("couldn't do read\n");
     }
@@ -253,6 +254,17 @@ sys_kill()
   return -1;
 }
 
+int
+sys_panic()
+{
+  struct proc *p = curproc[cpu()];
+  unsigned int addr;
+
+  fetcharg(0, &addr);
+  panic(p->mem + addr);
+  return 0;
+}
+
 void
 syscall()
 {
@@ -292,6 +304,9 @@ syscall()
   case SYS_kill:
     ret = sys_kill();
     break;
+  case SYS_panic:
+    ret = sys_panic();
+    break;
   default:
     cprintf("unknown sys call %d\n", num);
     // XXX fault

syscall.h

@@ -8,3 +8,4 @@
 #define SYS_close 8
 #define SYS_block 9
 #define SYS_kill 10
+#define SYS_panic 11

trap.c

@@ -36,8 +36,14 @@ trap(struct Trapframe *tf)
 {
   int v = tf->tf_trapno;
 
+  if(cpus[cpu()].clis){
+    cprintf("cpu %d v %d eip %x\n", cpu(), v, tf->tf_eip);
+    panic("interrupt while interrupts are off");
+  }
+
   if(v == T_SYSCALL){
     struct proc *cp = curproc[cpu()];
+    int num = cp->tf->tf_regs.reg_eax;
     if(cp == 0)
       panic("syscall with no proc");
     if(cp->killed)
@@ -50,6 +56,14 @@ trap(struct Trapframe *tf)
       panic("trap ret but not RUNNING");
     if(tf != cp->tf)
       panic("trap ret wrong tf");
+    if(cp->locks){
+      cprintf("num=%d\n", num);
+      panic("syscall returning locks held");
+    }
+    if(cpus[cpu()].clis)
+      panic("syscall returning but clis != 0");
+    if((read_eflags() & FL_IF) == 0)
+      panic("syscall returning but FL_IF clear");
     if(read_esp() < (unsigned)cp->kstack ||
        read_esp() >= (unsigned)cp->kstack + KSTACKSIZE)
       panic("trap ret esp wrong");
@@ -61,14 +75,20 @@ trap(struct Trapframe *tf)
   if(v == (IRQ_OFFSET + IRQ_TIMER)){
     struct proc *cp = curproc[cpu()];
     lapic_timerintr();
+    if(cp && cp->locks)
+      panic("timer interrupt while holding a lock");
     if(cp){
-      if(cpus[cpu()].clis != 0)
+#if 1
-        panic("trap clis > 0");
+      if((read_eflags() & FL_IF) == 0)
+        panic("timer interrupt but interrupts now disabled");
+#else
       cpus[cpu()].clis += 1;
       sti();
+#endif
       if(cp->killed)
         proc_exit();
-      yield();
+      if(cp->state == RUNNING)
+        yield();
     }
     return;
   }

usertests.c

@@ -16,7 +16,7 @@ pipe1()
       for(i = 0; i < 1033; i++)
         buf[i] = seq++;
       if(write(fds[1], buf, 1033) != 1033){
-        puts("pipe1 oops 1\n");
+        panic("pipe1 oops 1\n");
         exit(1);
       }
     }
@@ -31,7 +31,7 @@ pipe1()
         break;
       for(i = 0; i < n; i++){
         if((buf[i] & 0xff) != (seq++ & 0xff)){
-          puts("pipe1 oops 2\n");
+          panic("pipe1 oops 2\n");
           return;
         }
       }
@@ -41,8 +41,9 @@ pipe1()
         cc = sizeof(buf);
     }
     if(total != 5 * 1033)
-      puts("pipe1 oops 3\n");
+      panic("pipe1 oops 3\n");
     close(fds[0]);
+    wait();
   }
   puts("pipe1 ok\n");
 }
@@ -69,7 +70,7 @@ preempt()
   if(pid3 == 0){
     close(pfds[0]);
     if(write(pfds[1], "x", 1) != 1)
-      puts("preempt write error");
+      panic("preempt write error");
     close(pfds[1]);
     while(1)
       ;
@@ -77,7 +78,7 @@ preempt()
 
   close(pfds[1]);
   if(read(pfds[0], buf, sizeof(buf)) != 1){
-    puts("preempt read error");
+    panic("preempt read error");
     return;
   }
   close(pfds[0]);
@@ -90,12 +91,37 @@ preempt()
   puts("preempt ok\n");
 }
 
+// try to find any races between exit and wait
+void
+exitwait()
+{
+  int i, pid;
+
+  for(i = 0; i < 100; i++){
+    pid = fork();
+    if(pid < 0){
+      panic("fork failed\n");
+      return;
+    }
+    if(pid){
+      if(wait() != pid){
+        panic("wait wrong pid\n");
+        return;
+      }
+    } else {
+      exit(0);
+    }
+  }
+  puts("exitwait ok\n");
+}
+
 main()
 {
   puts("usertests starting\n");
-  pipe1();
-  //preempt();
 
-  while(1)
+  pipe1();
-    ;
+  preempt();
+  exitwait();
+
+  panic("usertests finished successfuly");
 }

usys.S

@@ -18,3 +18,4 @@ STUB(write)
 STUB(close)
 STUB(block)
 STUB(kill)
+STUB(panic)