pi-futex: Fix exit races and locking problems

1. New entries can be added to tsk->pi_state_list after task completed
   exit_pi_state_list(). The result is memory leakage and deadlocks.

2. handle_mm_fault() is called under spinlock. The result is obvious.

3. results in self-inflicted deadlock inside glibc.
   Sometimes futex_lock_pi returns -ESRCH, when it is not expected
   and glibc enters to for(;;) sleep() to simulate deadlock. This problem
   is quite obvious and I think the patch is right. Though it looks like
   each "if" in futex_lock_pi() got some stupid special case "else if". :-)

4. sometimes futex_lock_pi() returns -EDEADLK,
   when nobody has the lock. The reason is also obvious (see comment
   in the patch), but correct fix is far beyond my comprehension.
   I guess someone already saw this, the chunk:

                        if (rt_mutex_trylock(&q.pi_state->pi_mutex))
                                ret = 0;

   is obviously from the same opera. But it does not work, because the
   rtmutex is really taken at this point: wake_futex_pi() of previous
   owner reassigned it to us. My fix works. But it looks very stupid.
   I would think about removal of shift of ownership in wake_futex_pi()
   and making all the work in context of process taking lock.

From: Thomas Gleixner <tglx@linutronix.de>

Fix 1) Avoid the tasklist lock variant of the exit race fix by adding
    an additional state transition to the exit code.

    This fixes also the issue, when a task with recursive segfaults
    is not able to release the futexes.

Fix 2) Cleanup the lookup_pi_state() failure path and solve the -ESRCH
    problem finally.

Fix 3) Solve the fixup_pi_state_owner() problem which needs to do the fixup
    in the lock protected section by using the in_atomic userspace access
    functions.
	
    This removes also the ugly lock drop / unqueue inside of fixup_pi_state()

Fix 4) Fix a stale lock in the error path of futex_wake_pi()

Added some error checks for verification.

The -EDEADLK problem is solved by the rtmutex fixups.

Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/sched.h

@@ -1138,6 +1138,7 @@ static inline void put_task_struct(struct task_struct *t)
 					/* Not implemented yet, only for 486*/
 #define PF_STARTING	0x00000002	/* being created */
 #define PF_EXITING	0x00000004	/* getting shut down */
+#define PF_EXITPIDONE	0x00000008	/* pi exit done on shut down */
 #define PF_FORKNOEXEC	0x00000040	/* forked but didn't exec */
 #define PF_SUPERPRIV	0x00000100	/* used super-user privileges */
 #define PF_DUMPCORE	0x00000200	/* dumped core */

kernel/exit.c

@@ -884,13 +884,29 @@ fastcall NORET_TYPE void do_exit(long code)
 	if (unlikely(tsk->flags & PF_EXITING)) {
 		printk(KERN_ALERT
 			"Fixing recursive fault but reboot is needed!\n");
+		/*
+		 * We can do this unlocked here. The futex code uses
+		 * this flag just to verify whether the pi state
+		 * cleanup has been done or not. In the worst case it
+		 * loops once more. We pretend that the cleanup was
+		 * done as there is no way to return. Either the
+		 * OWNER_DIED bit is set by now or we push the blocked
+		 * task into the wait for ever nirwana as well.
+		 */
+		tsk->flags |= PF_EXITPIDONE;
 		if (tsk->io_context)
 			exit_io_context();
 		set_current_state(TASK_UNINTERRUPTIBLE);
 		schedule();
 	}
 
+	/*
+	 * tsk->flags are checked in the futex code to protect against
+	 * an exiting task cleaning up the robust pi futexes.
+	 */
+	spin_lock_irq(&tsk->pi_lock);
 	tsk->flags |= PF_EXITING;
+	spin_unlock_irq(&tsk->pi_lock);
 
 	if (unlikely(in_atomic()))
 		printk(KERN_INFO "note: %s[%d] exited with preempt_count %d\n",
@@ -957,6 +973,12 @@ fastcall NORET_TYPE void do_exit(long code)
 	 * Make sure we are holding no locks:
 	 */
 	debug_check_no_locks_held(tsk);
+	/*
+	 * We can do this unlocked here. The futex code uses this flag
+	 * just to verify whether the pi state cleanup has been done
+	 * or not. In the worst case it loops once more.
+	 */
+	tsk->flags |= PF_EXITPIDONE;
 
 	if (tsk->io_context)
 		exit_io_context();

kernel/futex.c

@@ -396,10 +396,6 @@ static struct task_struct * futex_find_get_task(pid_t pid)
 		p = NULL;
 		goto out_unlock;
 	}
-	if (p->exit_state != 0) {
-		p = NULL;
-		goto out_unlock;
-	}
 	get_task_struct(p);
 out_unlock:
 	rcu_read_unlock();
@@ -467,7 +463,7 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, struct futex_q *me)
 	struct futex_q *this, *next;
 	struct list_head *head;
 	struct task_struct *p;
-	pid_t pid;
+	pid_t pid = uval & FUTEX_TID_MASK;
 
 	head = &hb->chain;
 
@@ -485,6 +481,8 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, struct futex_q *me)
 				return -EINVAL;
 
 			WARN_ON(!atomic_read(&pi_state->refcount));
+			WARN_ON(pid && pi_state->owner &&
+				pi_state->owner->pid != pid);
 
 			atomic_inc(&pi_state->refcount);
 			me->pi_state = pi_state;
@@ -495,15 +493,33 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, struct futex_q *me)
 
 	/*
 	 * We are the first waiter - try to look up the real owner and attach
-	 * the new pi_state to it, but bail out when the owner died bit is set
-	 * and TID = 0:
+	 * the new pi_state to it, but bail out when TID = 0
 	 */
-	pid = uval & FUTEX_TID_MASK;
-	if (!pid && (uval & FUTEX_OWNER_DIED))
+	if (!pid)
 		return -ESRCH;
 	p = futex_find_get_task(pid);
-	if (!p)
-		return -ESRCH;
+	if (IS_ERR(p))
+		return PTR_ERR(p);
+
+	/*
+	 * We need to look at the task state flags to figure out,
+	 * whether the task is exiting. To protect against the do_exit
+	 * change of the task flags, we do this protected by
+	 * p->pi_lock:
+	 */
+	spin_lock_irq(&p->pi_lock);
+	if (unlikely(p->flags & PF_EXITING)) {
+		/*
+		 * The task is on the way out. When PF_EXITPIDONE is
+		 * set, we know that the task has finished the
+		 * cleanup:
+		 */
+		int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;
+
+		spin_unlock_irq(&p->pi_lock);
+		put_task_struct(p);
+		return ret;
+	}
 
 	pi_state = alloc_pi_state();
 
@@ -516,7 +532,6 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, struct futex_q *me)
 	/* Store the key for possible exit cleanups: */
 	pi_state->key = me->key;
 
-	spin_lock_irq(&p->pi_lock);
 	WARN_ON(!list_empty(&pi_state->list));
 	list_add(&pi_state->list, &p->pi_state_list);
 	pi_state->owner = p;
@@ -583,15 +598,22 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
 	 * preserve the owner died bit.)
 	 */
 	if (!(uval & FUTEX_OWNER_DIED)) {
+		int ret = 0;
+
 		newval = FUTEX_WAITERS | new_owner->pid;
 
 		pagefault_disable();
 		curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
 		pagefault_enable();
+
 		if (curval == -EFAULT)
-			return -EFAULT;
+			ret = -EFAULT;
 		if (curval != uval)
-			return -EINVAL;
+			ret = -EINVAL;
+		if (ret) {
+			spin_unlock(&pi_state->pi_mutex.wait_lock);
+			return ret;
+		}
 	}
 
 	spin_lock_irq(&pi_state->owner->pi_lock);
@@ -1149,6 +1171,7 @@ static int futex_lock_pi(u32 __user *uaddr, int detect, unsigned long sec,
 	if (unlikely(ret != 0))
 		goto out_release_sem;
 
+ retry_unlocked:
 	hb = queue_lock(&q, -1, NULL);
 
  retry_locked:
@@ -1200,34 +1223,58 @@ static int futex_lock_pi(u32 __user *uaddr, int detect, unsigned long sec,
 	ret = lookup_pi_state(uval, hb, &q);
 
 	if (unlikely(ret)) {
-		/*
-		 * There were no waiters and the owner task lookup
-		 * failed. When the OWNER_DIED bit is set, then we
-		 * know that this is a robust futex and we actually
-		 * take the lock. This is safe as we are protected by
-		 * the hash bucket lock. We also set the waiters bit
-		 * unconditionally here, to simplify glibc handling of
-		 * multiple tasks racing to acquire the lock and
-		 * cleanup the problems which were left by the dead
-		 * owner.
-		 */
-		if (curval & FUTEX_OWNER_DIED) {
-			uval = newval;
-			newval = current->pid |
-				FUTEX_OWNER_DIED | FUTEX_WAITERS;
+		switch (ret) {
 
-			pagefault_disable();
-			curval = futex_atomic_cmpxchg_inatomic(uaddr,
-							       uval, newval);
-			pagefault_enable();
+		case -EAGAIN:
+			/*
+			 * Task is exiting and we just wait for the
+			 * exit to complete.
+			 */
+			queue_unlock(&q, hb);
+			up_read(&curr->mm->mmap_sem);
+			cond_resched();
+			goto retry;
 
-			if (unlikely(curval == -EFAULT))
+		case -ESRCH:
+			/*
+			 * No owner found for this futex. Check if the
+			 * OWNER_DIED bit is set to figure out whether
+			 * this is a robust futex or not.
+			 */
+			if (get_futex_value_locked(&curval, uaddr))
 				goto uaddr_faulted;
-			if (unlikely(curval != uval))
-				goto retry_locked;
-			ret = 0;
+
+			/*
+			 * There were no waiters and the owner task lookup
+			 * failed. When the OWNER_DIED bit is set, then we
+			 * know that this is a robust futex and we actually
+			 * take the lock. This is safe as we are protected by
+			 * the hash bucket lock. We also set the waiters bit
+			 * unconditionally here, to simplify glibc handling of
+			 * multiple tasks racing to acquire the lock and
+			 * cleanup the problems which were left by the dead
+			 * owner.
+			 */
+			if (curval & FUTEX_OWNER_DIED) {
+				uval = newval;
+				newval = current->pid |
+					FUTEX_OWNER_DIED | FUTEX_WAITERS;
+
+				pagefault_disable();
+				curval = futex_atomic_cmpxchg_inatomic(uaddr,
+								       uval,
+								       newval);
+				pagefault_enable();
+
+				if (unlikely(curval == -EFAULT))
+					goto uaddr_faulted;
+				if (unlikely(curval != uval))
+					goto retry_locked;
+				ret = 0;
+			}
+		default:
+			goto out_unlock_release_sem;
 		}
-		goto out_unlock_release_sem;
 	}
 
 	/*
@@ -1279,39 +1326,52 @@ static int futex_lock_pi(u32 __user *uaddr, int detect, unsigned long sec,
 		list_add(&q.pi_state->list, &current->pi_state_list);
 		spin_unlock_irq(&current->pi_lock);
 
-		/* Unqueue and drop the lock */
-		unqueue_me_pi(&q, hb);
-		up_read(&curr->mm->mmap_sem);
 		/*
 		 * We own it, so we have to replace the pending owner
-		 * TID. This must be atomic as we have preserve the
+		 * TID. This must be atomic as we have to preserve the
 		 * owner died bit here.
 		 */
-		ret = get_user(uval, uaddr);
+		ret = get_futex_value_locked(&uval, uaddr);
 		while (!ret) {
 			newval = (uval & FUTEX_OWNER_DIED) | newtid;
+
+			pagefault_disable();
 			curval = futex_atomic_cmpxchg_inatomic(uaddr,
 							       uval, newval);
+			pagefault_enable();
+
 			if (curval == -EFAULT)
 				ret = -EFAULT;
 			if (curval == uval)
 				break;
 			uval = curval;
 		}
-	} else {
+	} else if (ret) {
 		/*
 		 * Catch the rare case, where the lock was released
 		 * when we were on the way back before we locked
 		 * the hash bucket.
 		 */
-		if (ret && q.pi_state->owner == curr) {
-			if (rt_mutex_trylock(&q.pi_state->pi_mutex))
-				ret = 0;
+		if (q.pi_state->owner == curr &&
+		    rt_mutex_trylock(&q.pi_state->pi_mutex)) {
+			ret = 0;
+		} else {
+			/*
+			 * Paranoia check. If we did not take the lock
+			 * in the trylock above, then we should not be
+			 * the owner of the rtmutex, neither the real
+			 * nor the pending one:
+			 */
+			if (rt_mutex_owner(&q.pi_state->pi_mutex) == curr)
+				printk(KERN_ERR "futex_lock_pi: ret = %d "
+				       "pi-mutex: %p pi-state %p\n", ret,
+				       q.pi_state->pi_mutex.owner,
+				       q.pi_state->owner);
 		}
-		/* Unqueue and drop the lock */
-		unqueue_me_pi(&q, hb);
-		up_read(&curr->mm->mmap_sem);
 	}
+	/* Unqueue and drop the lock */
+	unqueue_me_pi(&q, hb);
+	up_read(&curr->mm->mmap_sem);
 
 	if (!detect && ret == -EDEADLK && 0)
 		force_sig(SIGKILL, current);
@@ -1331,16 +1391,18 @@ static int futex_lock_pi(u32 __user *uaddr, int detect, unsigned long sec,
 	 * non-atomically.  Therefore, if get_user below is not
 	 * enough, we need to handle the fault ourselves, while
 	 * still holding the mmap_sem.
+	 *
+	 * ... and hb->lock. :-) --ANK
 	 */
+	queue_unlock(&q, hb);
+
 	if (attempt++) {
-		if (futex_handle_fault((unsigned long)uaddr, attempt)) {
-			ret = -EFAULT;
-			goto out_unlock_release_sem;
-		}
-		goto retry_locked;
+		ret = futex_handle_fault((unsigned long)uaddr, attempt);
+		if (ret)
+			goto out_release_sem;
+		goto retry_unlocked;
 	}
 
-	queue_unlock(&q, hb);
 	up_read(&curr->mm->mmap_sem);
 
 	ret = get_user(uval, uaddr);
@@ -1382,9 +1444,9 @@ static int futex_unlock_pi(u32 __user *uaddr)
 		goto out;
 
 	hb = hash_futex(&key);
+retry_unlocked:
 	spin_lock(&hb->lock);
 
-retry_locked:
 	/*
 	 * To avoid races, try to do the TID -> 0 atomic transition
 	 * again. If it succeeds then we can return without waking
@@ -1446,16 +1508,17 @@ static int futex_unlock_pi(u32 __user *uaddr)
 	 * non-atomically.  Therefore, if get_user below is not
 	 * enough, we need to handle the fault ourselves, while
 	 * still holding the mmap_sem.
+	 *
+	 * ... and hb->lock. :-) --ANK
 	 */
+	spin_unlock(&hb->lock);
+
 	if (attempt++) {
-		if (futex_handle_fault((unsigned long)uaddr, attempt)) {
-			ret = -EFAULT;
-			goto out_unlock;
-		}
-		goto retry_locked;
+		ret = futex_handle_fault((unsigned long)uaddr, attempt);
+		if (ret)
+			goto out;
+		goto retry_unlocked;
 	}
-
-	spin_unlock(&hb->lock);
 	up_read(&current->mm->mmap_sem);
 
 	ret = get_user(uval, uaddr);