linux-kernel - [PATCH 4.9.257 1/1] futex: Fix OWNER

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Date:   Mon, 22 Feb 2021 19:05:42 +0800
From:   Zheng Yejian <zhengyejian1@...wei.com>
To:     <gregkh@...uxfoundation.org>, <stable@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC:     <judy.chenhui@...wei.com>, <zhangjinhao2@...wei.com>,
        <lee.jones@...aro.org>, <tglx@...utronix.de>
Subject: [PATCH 4.9.257 1/1] futex: Fix OWNER_DEAD fixup

From: Peter Zijlstra <peterz@...radead.org>

commit a97cb0e7b3f4c6297fd857055ae8e895f402f501 upstream.

Both Geert and DaveJ reported that the recent futex commit:

  c1e2f0eaf015 ("futex: Avoid violating the 10th rule of futex")

introduced a problem with setting OWNER_DEAD. We set the bit on an
uninitialized variable and then entirely optimize it away as a
dead-store.

Move the setting of the bit to where it is more useful.

Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Reported-by: Dave Jones <davej@...emonkey.org.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Fixes: c1e2f0eaf015 ("futex: Avoid violating the 10th rule of futex")
Link: http://lkml.kernel.org/r/20180122103947.GD2228@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Zheng Yejian <zhengyejian1@...wei.com>
---
 kernel/futex.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 83db5787c67e..7cb2baa9446a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2283,10 +2283,6 @@ static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
 
 	oldowner = pi_state->owner;
 
-	/* Owner died? */
-	if (!pi_state->owner)
-		newtid |= FUTEX_OWNER_DIED;
-
 	/*
 	 * We are here because either:
 	 *
@@ -2344,6 +2340,9 @@ static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
 	}
 
 	newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
+	/* Owner died? */
+	if (!pi_state->owner)
+		newtid |= FUTEX_OWNER_DIED;
 
 	if (get_futex_value_locked(&uval, uaddr))
 		goto handle_fault;
-- 
2.25.4