linux-kernel - [patch 11/12] futex: Provide distinct return value when owner is exiting

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Message-Id: <20191106224556.935606117@linutronix.de>
Date:   Wed, 06 Nov 2019 22:55:45 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Darren Hart <darren@...art.com>,
        Yi Wang <wang.yi59@....com.cn>,
        Yang Tao <yang.tao172@....com.cn>,
        Oleg Nesterov <oleg@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Carlos O'Donell <carlos@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: [patch 11/12] 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 funcitonal change.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/futex.c |   16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1182,11 +1182,11 @@ static int handle_exit_race(u32 __user *
 	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 @@ static int futex_requeue(u32 __user *uad
 			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 @@ static int futex_lock_pi(u32 __user *uad
 			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);