linux-kernel - [PATCH stable-rc queue/4.9 1/1] futex: Provide distinct return value when owner is exiting

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Message-ID: <20210222070328.102384-2-nixiaoming@huawei.com>
Date:   Mon, 22 Feb 2021 15:03:28 +0800
From:   Xiaoming Ni <nixiaoming@...wei.com>
To:     <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        <gregkh@...uxfoundation.org>, <sashal@...nel.org>,
        <tglx@...utronix.de>
CC:     <nixiaoming@...wei.com>, <wangle6@...wei.com>,
        <zhengyejian1@...wei.com>
Subject: [PATCH stable-rc queue/4.9 1/1] futex: Provide distinct return value when owner is exiting

From: Thomas Gleixner <tglx@...utronix.de>

commit ac31c7ff8624409ba3c4901df9237a616c187a5d 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@...utronix.de>
Reviewed-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20191106224556.935606117@linutronix.de

[nixiaoming: Modify handle_exit_race() to avoid dead code.]
Cc: stable@...r.kernel.org # queue/4.9
Signed-off-by: Xiaoming Ni <nixiaoming@...wei.com>
---
 kernel/futex.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index b65dbb5d60bb..0fd785410150 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1207,11 +1207,11 @@ 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:
-- 
2.27.0