futex: Provide distinct return value when owner is exiting

commit ac31c7ff upstream.

attach_to_pi_owner() returns -EAGAIN for various cases:

 - Owner task is exiting
 - Futex value has changed

The caller drops the held locks (hash bucket, mmap_sem) and retries the
operation. In case of the owner task exiting this can result in a live
lock.

As a preparatory step for seperating those cases, provide a distinct return
value (EBUSY) for the owner exiting case.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.935606117@linutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/futex.c

@@ -1909,12 +1909,13 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
 			if (!ret)
 				goto retry;
 			goto out;
+		case -EBUSY:
 		case -EAGAIN:
 			/*
 			 * Two reasons for this:
-			 * - Owner is exiting and we just wait for the
+			 * - EBUSY: Owner is exiting and we just wait for the
 			 *   exit to complete.
-			 * - The user space value changed.
+			 * - EAGAIN: The user space value changed.
 			 */
 			free_pi_state(pi_state);
 			pi_state = NULL;
@@ -2580,12 +2581,13 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
 			goto out_unlock_put_key;
 		case -EFAULT:
 			goto uaddr_faulted;
+		case -EBUSY:
 		case -EAGAIN:
 			/*
 			 * Two reasons for this:
-			 * - Task is exiting and we just wait for the
+			 * - EBUSY: Task is exiting and we just wait for the
 			 *   exit to complete.
-			 * - The user space value changed.
+			 * - EAGAIN: The user space value changed.
 			 */
 			queue_unlock(hb);
 			put_futex_key(&q.key);