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From:   gregkh@linuxfoundation.org
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Xiaoming Ni <nixiaoming@huawei.com>,
        Lee Jones <lee.jones@linaro.org>,
        Zheng Yejian <zhengyejian1@huawei.com>
Subject: [PATCH 4.4 13/75] futex: fix dead code in attach_to_pi_owner()
Date:   Mon, 15 Mar 2021 14:51:27 +0100
Message-Id: <20210315135208.696700074@linuxfoundation.org>
In-Reply-To: <20210315135208.252034256@linuxfoundation.org>
References: <20210315135208.252034256@linuxfoundation.org>
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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

From: Thomas Gleixner <tglx@linutronix.de>

This patch comes directly from an origin patch (commit
91509e84949fc97e7424521c32a9e227746e0b85) in v4.9.
And it is part of a full patch which was originally back-ported
to v4.14 as commit e6e00df182908f34360c3c9f2d13cc719362e9c0

The handle_exit_race() function is defined in commit 9c3f39860367
 ("futex: Cure exit race"), which never returns -EBUSY. This results
in a small piece of dead code in the attach_to_pi_owner() function:

	int ret = handle_exit_race(uaddr, uval, p); /* Never return -EBUSY */
	...
	if (ret == -EBUSY)
		*exiting = p; /* dead code */

The return value -EBUSY is added to handle_exit_race() in upsteam
commit ac31c7ff8624409 ("futex: Provide distinct return value when
owner is exiting"). This commit was incorporated into v4.9.255, before
the function handle_exit_race() was introduced, whitout Modify
handle_exit_race().

To fix dead code, extract the change of handle_exit_race() from
commit ac31c7ff8624409 ("futex: Provide distinct return value when owner
 is exiting"), re-incorporated.

Lee writes:

This commit takes the remaining functional snippet of:

 ac31c7ff8624409 ("futex: Provide distinct return value when owner is exiting")

... and is the correct fix for this issue.

Fixes: 9c3f39860367 ("futex: Cure exit race")
Cc: stable@vger.kernel.org # v4.9.258
Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Reviewed-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/futex.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1204,11 +1204,11 @@ 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: