Merge branch 'core-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'core-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  futexes: fix fault handling in futex_lock_pi

1 files changed, 73 insertions, 20 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 449def8074fe..7d1136e97c14 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1096,21 +1096,64 @@ static void unqueue_me_pi(struct futex_q *q)
  * private futexes.
  */
 static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
-                                struct task_struct *newowner)
+                                struct task_struct *newowner,
+                                struct rw_semaphore *fshared)
 {
         u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
         struct futex_pi_state *pi_state = q->pi_state;
+        struct task_struct *oldowner = pi_state->owner;
         u32 uval, curval, newval;
-        int ret;
+        int ret, attempt = 0;
 
         /* Owner died? */
+        if (!pi_state->owner)
+                newtid |= FUTEX_OWNER_DIED;
+
+        /*
+         * We are here either because we stole the rtmutex from the
+         * pending owner or we are the pending owner which failed to
+         * get the rtmutex. We have to replace the pending owner TID
+         * in the user space variable. This must be atomic as we have
+         * to preserve the owner died bit here.
+         *
+         * Note: We write the user space value _before_ changing the
+         * pi_state because we can fault here. Imagine swapped out
+         * pages or a fork, which was running right before we acquired
+         * mmap_sem, that marked all the anonymous memory readonly for
+         * cow.
+         *
+         * Modifying pi_state _before_ the user space value would
+         * leave the pi_state in an inconsistent state when we fault
+         * here, because we need to drop the hash bucket lock to
+         * handle the fault. This might be observed in the PID check
+         * in lookup_pi_state.
+         */
+retry:
+        if (get_futex_value_locked(&uval, uaddr))
+                goto handle_fault;
+
+        while (1) {
+                newval = (uval & FUTEX_OWNER_DIED) | newtid;
+
+                curval = cmpxchg_futex_value_locked(uaddr, uval, newval);
+
+                if (curval == -EFAULT)
+                        goto handle_fault;
+                if (curval == uval)
+                        break;
+                uval = curval;
+        }
+
+        /*
+         * We fixed up user space. Now we need to fix the pi_state
+         * itself.
+         */
         if (pi_state->owner != NULL) {
                 spin_lock_irq(&pi_state->owner->pi_lock);
                 WARN_ON(list_empty(&pi_state->list));
                 list_del_init(&pi_state->list);
                 spin_unlock_irq(&pi_state->owner->pi_lock);
-        } else
-                newtid |= FUTEX_OWNER_DIED;
+        }
 
         pi_state->owner = newowner;
 
@@ -1118,26 +1161,35 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
         WARN_ON(!list_empty(&pi_state->list));
         list_add(&pi_state->list, &newowner->pi_state_list);
         spin_unlock_irq(&newowner->pi_lock);
+        return 0;
 
         /*
-         * We own it, so we have to replace the pending owner
-         * TID. This must be atomic as we have preserve the
-         * owner died bit here.
+         * To handle the page fault we need to drop the hash bucket
+         * lock here. That gives the other task (either the pending
+         * owner itself or the task which stole the rtmutex) the
+         * chance to try the fixup of the pi_state. So once we are
+         * back from handling the fault we need to check the pi_state
+         * after reacquiring the hash bucket lock and before trying to
+         * do another fixup. When the fixup has been done already we
+         * simply return.
          */
-        ret = get_futex_value_locked(&uval, uaddr);
+handle_fault:
+        spin_unlock(q->lock_ptr);
 
-        while (!ret) {
-                newval = (uval & FUTEX_OWNER_DIED) | newtid;
+        ret = futex_handle_fault((unsigned long)uaddr, fshared, attempt++);
 
-                curval = cmpxchg_futex_value_locked(uaddr, uval, newval);
+        spin_lock(q->lock_ptr);
 
-                if (curval == -EFAULT)
-                        ret = -EFAULT;
-                if (curval == uval)
-                        break;
-                uval = curval;
-        }
-        return ret;
+        /*
+         * Check if someone else fixed it for us:
+         */
+        if (pi_state->owner != oldowner)
+                return 0;
+
+        if (ret)
+                return ret;
+
+        goto retry;
 }
 
 /*
@@ -1507,7 +1559,7 @@ static int futex_lock_pi(u32 __user *uaddr, struct rw_semaphore *fshared,
                  * that case:
                  */
                 if (q.pi_state->owner != curr)
-                        ret = fixup_pi_state_owner(uaddr, &q, curr);
+                        ret = fixup_pi_state_owner(uaddr, &q, curr, fshared);
         } else {
                 /*
                  * Catch the rare case, where the lock was released
@@ -1539,7 +1591,8 @@ static int futex_lock_pi(u32 __user *uaddr, struct rw_semaphore *fshared,
                                 int res;
 
                                 owner = rt_mutex_owner(&q.pi_state->pi_mutex);
-                                res = fixup_pi_state_owner(uaddr, &q, owner);
+                                res = fixup_pi_state_owner(uaddr, &q, owner,
+                                                           fshared);
 
                                 /* propagate -EFAULT, if the fixup failed */
                                 if (res)