futex: fix restart in wait_requeue_pi

If the waiter has been requeued to the outer PI futex and is
interrupted by a signal and the thread handles the signal then
ERESTART_RESTARTBLOCK is changed to EINTR and the restart block is
discarded. That way we return an unexcpected EINTR to user space
instead of ending up in futex_lock_pi_restart.

But we do not need to restart the syscall because we know that the
condition has changed since we have been requeued. If we would simply
restart the syscall then we would drop out via the comparison of the
user space value with EWOULDBLOCK.

The user space side needs to handle EWOULDBLOCK anyway as the
enqueueing on the inner futex can race with a requeue/wake. So we can
simply return EWOULDBLOCK to user space which also signals that we did
not take the outer futex and let user space handle it in the same way
it has to handle the requeue/wake race.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/futex.c

@@ -1507,7 +1507,6 @@ handle_fault:
 #define FLAGS_HAS_TIMEOUT	0x04
 
 static long futex_wait_restart(struct restart_block *restart);
-static long futex_lock_pi_restart(struct restart_block *restart);
 
 /**
  * fixup_owner() - Post lock pi_state and corner case management
@@ -1930,21 +1929,6 @@ uaddr_faulted:
 	goto retry;
 }
 
-static long futex_lock_pi_restart(struct restart_block *restart)
-{
-	u32 __user *uaddr = (u32 __user *)restart->futex.uaddr;
-	ktime_t t, *tp = NULL;
-	int fshared = restart->futex.flags & FLAGS_SHARED;
-
-	if (restart->futex.flags & FLAGS_HAS_TIMEOUT) {
-		t.tv64 = restart->futex.time;
-		tp = &t;
-	}
-	restart->fn = do_no_restart_syscall;
-
-	return (long)futex_lock_pi(uaddr, fshared, restart->futex.val, tp, 0);
-}
-
 /*
  * Userspace attempted a TID -> 0 atomic transition, and failed.
  * This is the in-kernel slowpath: we look up the PI state (if any),
@@ -2141,12 +2125,10 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
 	struct hrtimer_sleeper timeout, *to = NULL;
 	struct rt_mutex_waiter rt_waiter;
 	struct rt_mutex *pi_mutex = NULL;
-	struct restart_block *restart;
 	struct futex_hash_bucket *hb;
 	union futex_key key2;
 	struct futex_q q;
 	int res, ret;
-	u32 uval;
 
 	if (!bitset)
 		return -EINVAL;
@@ -2245,30 +2227,17 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
 		if (rt_mutex_owner(pi_mutex) == current)
 			rt_mutex_unlock(pi_mutex);
 	} else if (ret == -EINTR) {
-		ret = -EFAULT;
-		if (get_user(uval, uaddr2))
-			goto out_put_keys;
-
 		/*
-		 * We've already been requeued, so restart by calling
-		 * futex_lock_pi() directly, rather then returning to this
-		 * function.
+		 * We've already been requeued, but we have no way to
+		 * restart by calling futex_lock_pi() directly. We
+		 * could restart the syscall, but that will look at
+		 * the user space value and return right away. So we
+		 * drop back with EWOULDBLOCK to tell user space that
+		 * "val" has been changed. That's the same what the
+		 * restart of the syscall would do in
+		 * futex_wait_setup().
 		 */
-		ret = -ERESTART_RESTARTBLOCK;
-		restart = &current_thread_info()->restart_block;
-		restart->fn = futex_lock_pi_restart;
-		restart->futex.uaddr = (u32 *)uaddr2;
-		restart->futex.val = uval;
-		restart->futex.flags = 0;
-		if (abs_time) {
-			restart->futex.flags |= FLAGS_HAS_TIMEOUT;
-			restart->futex.time = abs_time->tv64;
-		}
-
-		if (fshared)
-			restart->futex.flags |= FLAGS_SHARED;
-		if (clockrt)
-			restart->futex.flags |= FLAGS_CLOCKRT;
+		ret = -EWOULDBLOCK;
 	}
 
 out_put_keys: