[PATCH] pi-futex: robust-futex exit

Fix robust PI-futexes to be properly unlocked on unexpected exit.

For this to work the kernel has to know whether a futex is a PI or a
non-PI one, because the semantics are different.  Since the space in
relevant glibc data structures is extremely scarce, the best solution is
to encode the 'PI' information in bit 0 of the robust list pointer.
Existing (non-PI) glibc robust futexes have this bit always zero, so the
ABI is kept.  New glibc with PI-robust-futexes will set this bit.

Further fixes from Thomas Gleixner <tglx@linutronix.de>

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 87 insertions, 38 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index f59003b1d8f9..dda2049692a2 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -495,10 +495,13 @@ 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:
+         * 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:
          */
         pid = uval & FUTEX_TID_MASK;
+        if (!pid && (uval & FUTEX_OWNER_DIED))
+                return -ESRCH;
         p = futex_find_get_task(pid);
         if (!p)
                 return -ESRCH;
@@ -579,16 +582,17 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
          * kept enabled while there is PI state around. We must also
          * preserve the owner died bit.)
          */
-        newval = (uval & FUTEX_OWNER_DIED) | FUTEX_WAITERS | new_owner->pid;
-
-        inc_preempt_count();
-        curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
-        dec_preempt_count();
+        if (!(uval & FUTEX_OWNER_DIED)) {
+                newval = FUTEX_WAITERS | new_owner->pid;
 
-        if (curval == -EFAULT)
-                return -EFAULT;
-        if (curval != uval)
-                return -EINVAL;
+                inc_preempt_count();
+                curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
+                dec_preempt_count();
+                if (curval == -EFAULT)
+                        return -EFAULT;
+                if (curval != uval)
+                        return -EINVAL;
+        }
 
         spin_lock_irq(&pi_state->owner->pi_lock);
         WARN_ON(list_empty(&pi_state->list));
@@ -1443,9 +1447,11 @@ retry_locked:
          * again. If it succeeds then we can return without waking
          * anyone else up:
          */
-        inc_preempt_count();
-        uval = futex_atomic_cmpxchg_inatomic(uaddr, current->pid, 0);
-        dec_preempt_count();
+        if (!(uval & FUTEX_OWNER_DIED)) {
+                inc_preempt_count();
+                uval = futex_atomic_cmpxchg_inatomic(uaddr, current->pid, 0);
+                dec_preempt_count();
+        }
 
         if (unlikely(uval == -EFAULT))
                 goto pi_faulted;
@@ -1478,9 +1484,11 @@ retry_locked:
         /*
          * No waiters - kernel unlocks the futex:
          */
-        ret = unlock_futex_pi(uaddr, uval);
-        if (ret == -EFAULT)
-                goto pi_faulted;
+        if (!(uval & FUTEX_OWNER_DIED)) {
+                ret = unlock_futex_pi(uaddr, uval);
+                if (ret == -EFAULT)
+                        goto pi_faulted;
+        }
 
 out_unlock:
         spin_unlock(&hb->lock);
@@ -1699,9 +1707,9 @@ err_unlock:
  * Process a futex-list entry, check whether it's owned by the
  * dying task, and do notification if so:
  */
-int handle_futex_death(u32 __user *uaddr, struct task_struct *curr)
+int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
 {
-        u32 uval, nval;
+        u32 uval, nval, mval;
 
 retry:
         if (get_user(uval, uaddr))
@@ -1718,21 +1726,45 @@ retry:
                  * thread-death.) The rest of the cleanup is done in
                  * userspace.
                  */
-                nval = futex_atomic_cmpxchg_inatomic(uaddr, uval,
-                                                     uval | FUTEX_OWNER_DIED);
+                mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
+                nval = futex_atomic_cmpxchg_inatomic(uaddr, uval, mval);
+
                 if (nval == -EFAULT)
                         return -1;
 
                 if (nval != uval)
                         goto retry;
 
-                if (uval & FUTEX_WAITERS)
-                        futex_wake(uaddr, 1);
+                /*
+                 * Wake robust non-PI futexes here. The wakeup of
+                 * PI futexes happens in exit_pi_state():
+                 */
+                if (!pi) {
+                        if (uval & FUTEX_WAITERS)
+                                futex_wake(uaddr, 1);
+                }
         }
         return 0;
 }
 
 /*
+ * Fetch a robust-list pointer. Bit 0 signals PI futexes:
+ */
+static inline int fetch_robust_entry(struct robust_list __user **entry,
+                                     struct robust_list __user **head, int *pi)
+{
+        unsigned long uentry;
+
+        if (get_user(uentry, (unsigned long *)head))
+                return -EFAULT;
+
+        *entry = (void *)(uentry & ~1UL);
+        *pi = uentry & 1;
+
+        return 0;
+}
+
+/*
  * Walk curr->robust_list (very carefully, it's a userspace list!)
  * and mark any locks found there dead, and notify any waiters.
  *
@@ -1742,14 +1774,14 @@ void exit_robust_list(struct task_struct *curr)
 {
         struct robust_list_head __user *head = curr->robust_list;
         struct robust_list __user *entry, *pending;
-        unsigned int limit = ROBUST_LIST_LIMIT;
+        unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
         unsigned long futex_offset;
 
         /*
          * Fetch the list head (which was registered earlier, via
          * sys_set_robust_list()):
          */
-        if (get_user(entry, &head->list.next))
+        if (fetch_robust_entry(&entry, &head->list.next, &pi))
                 return;
         /*
          * Fetch the relative futex offset:
@@ -1760,10 +1792,11 @@ void exit_robust_list(struct task_struct *curr)
          * Fetch any possibly pending lock-add first, and handle it
          * if it exists:
          */
-        if (get_user(pending, &head->list_op_pending))
+        if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
                 return;
+
         if (pending)
-                handle_futex_death((void *)pending + futex_offset, curr);
+                handle_futex_death((void *)pending + futex_offset, curr, pip);
 
         while (entry != &head->list) {
                 /*
@@ -1772,12 +1805,12 @@ void exit_robust_list(struct task_struct *curr)
                  */
                 if (entry != pending)
                         if (handle_futex_death((void *)entry + futex_offset,
-                                                curr))
+                                                curr, pi))
                                 return;
                 /*
                  * Fetch the next entry in the list:
                  */
-                if (get_user(entry, &entry->next))
+                if (fetch_robust_entry(&entry, &entry->next, &pi))
                         return;
                 /*
                  * Avoid excessively long or circular lists:
diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index d1d92b441fb7..d1aab1a452cc 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -12,6 +12,23 @@
 
 #include <asm/uaccess.h>
 
+
+/*
+ * Fetch a robust-list pointer. Bit 0 signals PI futexes:
+ */
+static inline int
+fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
+                   compat_uptr_t *head, int *pi)
+{
+        if (get_user(*uentry, head))
+                return -EFAULT;
+
+        *entry = compat_ptr((*uentry) & ~1);
+        *pi = (unsigned int)(*uentry) & 1;
+
+        return 0;
+}
+
 /*
  * Walk curr->robust_list (very carefully, it's a userspace list!)
  * and mark any locks found there dead, and notify any waiters.
@@ -22,17 +39,16 @@ void compat_exit_robust_list(struct task_struct *curr)
 {
         struct compat_robust_list_head __user *head = curr->compat_robust_list;
         struct robust_list __user *entry, *pending;
+        unsigned int limit = ROBUST_LIST_LIMIT, pi;
         compat_uptr_t uentry, upending;
-        unsigned int limit = ROBUST_LIST_LIMIT;
         compat_long_t futex_offset;
 
         /*
          * Fetch the list head (which was registered earlier, via
          * sys_set_robust_list()):
          */
-        if (get_user(uentry, &head->list.next))
+        if (fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
                 return;
-        entry = compat_ptr(uentry);
         /*
          * Fetch the relative futex offset:
          */
@@ -42,11 +58,11 @@ void compat_exit_robust_list(struct task_struct *curr)
          * Fetch any possibly pending lock-add first, and handle it
          * if it exists:
          */
-        if (get_user(upending, &head->list_op_pending))
+        if (fetch_robust_entry(&upending, &pending,
+                               &head->list_op_pending, &pi))
                 return;
-        pending = compat_ptr(upending);
         if (upending)
-                handle_futex_death((void *)pending + futex_offset, curr);
+                handle_futex_death((void *)pending + futex_offset, curr, pi);
 
         while (compat_ptr(uentry) != &head->list) {
                 /*
@@ -55,15 +71,15 @@ void compat_exit_robust_list(struct task_struct *curr)
                  */
                 if (entry != pending)
                         if (handle_futex_death((void *)entry + futex_offset,
-                                                curr))
+                                                curr, pi))
                                 return;
 
                 /*
                  * Fetch the next entry in the list:
                  */
-                if (get_user(uentry, (compat_uptr_t *)&entry->next))
+                if (fetch_robust_entry(&uentry, &entry,
+                                       (compat_uptr_t *)&entry->next, &pi))
                         return;
-                entry = compat_ptr(uentry);
                 /*
                  * Avoid excessively long or circular lists:
                  */