Commit ac31c7ff authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

futex: Provide distinct return value when owner is exiting 



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: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.935606117@linutronix.de
parent 3f186d97

kernel/futex.c

+9 −7
Original line number Diff line number Diff line
@@ -1182,11 +1182,11 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval, 
	u32 uval2;



	/*

	 * If the futex exit state is not yet FUTEX_STATE_DEAD, wait

	 * for it to finish.

	 * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the

	 * caller that the alleged owner is busy.

	 */

	if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)

		return -EAGAIN;

		return -EBUSY;



	/*

	 * Reread the user space value to handle the following situation:
@@ -2092,12 +2092,13 @@ retry_private: 
			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.

			 */

			double_unlock_hb(hb1, hb2);

			hb_waiters_dec(hb2);
@@ -2843,12 +2844,13 @@ retry_private: 
			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);

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