futex: fixup unlocked requeue pi case

Thomas's testing caught a problem when the requeue target futex is
unowned and multiple tasks are requeued to it. This patch ensures
the FUTEX_WAITERS bit gets set if futex_requeue() will requeue one
or more tasks in addition to the one acquiring the lock.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/futex.c b/kernel/futex.c
index 185c981..041bf3a 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -565,12 +565,14 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
 
 /**
  * futex_lock_pi_atomic() - atomic work required to acquire a pi aware futex
- * @uaddr:     the pi futex user address
- * @hb:                the pi futex hash bucket
- * @key:       the futex key associated with uaddr and hb
- * @ps:                the pi_state pointer where we store the result of the lookup
- * @task:      the task to perform the atomic lock work for.  This will be
- *             "current" except in the case of requeue pi.
+ * @uaddr:             the pi futex user address
+ * @hb:                        the pi futex hash bucket
+ * @key:               the futex key associated with uaddr and hb
+ * @ps:                        the pi_state pointer where we store the result of the
+ *                     lookup
+ * @task:              the task to perform the atomic lock work for.  This will
+ *                     be "current" except in the case of requeue pi.
+ * @set_waiters:       force setting the FUTEX_WAITERS bit (1) or not (0)
  *
  * Returns:
  *  0 - ready to wait
@@ -582,7 +584,7 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
 static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
                                union futex_key *key,
                                struct futex_pi_state **ps,
-                               struct task_struct *task)
+                               struct task_struct *task, int set_waiters)
 {
        int lock_taken, ret, ownerdied = 0;
        u32 uval, newval, curval;
@@ -596,6 +598,8 @@ retry:
         * the locks. It will most likely not succeed.
         */
        newval = task_pid_vnr(task);
+       if (set_waiters)
+               newval |= FUTEX_WAITERS;
 
        curval = cmpxchg_futex_value_locked(uaddr, 0, newval);
 
@@ -1004,14 +1008,18 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key)
 
 /**
  * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
- * @pifutex:   the user address of the to futex
- * @hb1:       the from futex hash bucket, must be locked by the caller
- * @hb2:       the to futex hash bucket, must be locked by the caller
- * @key1:      the from futex key
- * @key2:      the to futex key
+ * @pifutex:           the user address of the to futex
+ * @hb1:               the from futex hash bucket, must be locked by the caller
+ * @hb2:               the to futex hash bucket, must be locked by the caller
+ * @key1:              the from futex key
+ * @key2:              the to futex key
+ * @ps:                        address to store the pi_state pointer
+ * @set_waiters:       force setting the FUTEX_WAITERS bit (1) or not (0)
  *
  * Try and get the lock on behalf of the top waiter if we can do it atomically.
- * Wake the top waiter if we succeed.  hb1 and hb2 must be held by the caller.
+ * Wake the top waiter if we succeed.  If the caller specified set_waiters,
+ * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
+ * hb1 and hb2 must be held by the caller.
  *
  * Returns:
  *  0 - failed to acquire the lock atomicly
@@ -1022,15 +1030,23 @@ static int futex_proxy_trylock_atomic(u32 __user *pifutex,
                                 struct futex_hash_bucket *hb1,
                                 struct futex_hash_bucket *hb2,
                                 union futex_key *key1, union futex_key *key2,
-                                struct futex_pi_state **ps)
+                                struct futex_pi_state **ps, int set_waiters)
 {
-       struct futex_q *top_waiter;
+       struct futex_q *top_waiter = NULL;
        u32 curval;
        int ret;
 
        if (get_futex_value_locked(&curval, pifutex))
                return -EFAULT;
 
+       /*
+        * Find the top_waiter and determine if there are additional waiters.
+        * If the caller intends to requeue more than 1 waiter to pifutex,
+        * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
+        * as we have means to handle the possible fault.  If not, don't set
+        * the bit unecessarily as it will force the subsequent unlock to enter
+        * the kernel.
+        */
        top_waiter = futex_top_waiter(hb1, key1);
 
        /* There are no waiters, nothing for us to do. */
@@ -1038,10 +1054,12 @@ static int futex_proxy_trylock_atomic(u32 __user *pifutex,
                return 0;
 
        /*
-        * Either take the lock for top_waiter or set the FUTEX_WAITERS bit.
-        * The pi_state is returned in ps in contended cases.
+        * Try to take the lock for top_waiter.  Set the FUTEX_WAITERS bit in
+        * the contended case or if set_waiters is 1.  The pi_state is returned
+        * in ps in contended cases.
         */
-       ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task);
+       ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
+                                  set_waiters);
        if (ret == 1)
                requeue_pi_wake_futex(top_waiter, key2);
 
@@ -1146,9 +1164,14 @@ retry_private:
        }
 
        if (requeue_pi && (task_count - nr_wake < nr_requeue)) {
-               /* Attempt to acquire uaddr2 and wake the top_waiter. */
+               /*
+                * Attempt to acquire uaddr2 and wake the top waiter. If we
+                * intend to requeue waiters, force setting the FUTEX_WAITERS
+                * bit.  We force this here where we are able to easily handle
+                * faults rather in the requeue loop below.
+                */
                ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
-                                                &key2, &pi_state);
+                                                &key2, &pi_state, nr_requeue);
 
                /*
                 * At this point the top_waiter has either taken uaddr2 or is
@@ -1810,7 +1833,7 @@ retry:
 retry_private:
        hb = queue_lock(&q);
 
-       ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current);
+       ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0);
        if (unlikely(ret)) {
                switch (ret) {
                case 1: