Re: [PATCH 4/4] futex: warning corrections

[Vitaliy Ivanov <vitalivanov@gmail.com>]

On Wed, 2011-07-06 at 10:00 -0700, Darren Hart wrote:
> On 07/04/2011 04:21 PM, Vitaliy Ivanov wrote:
> > From 3f7997d71fe2b5cb16e2913928f68023855d786d Mon Sep 17 00:00:00 2001
> > From: Vitaliy Ivanov <vitalivanov@gmail.com>
> > Date: Tue, 5 Jul 2011 02:07:42 +0300
> > Subject: [PATCH 4/4] futex: warning corrections
> Hi Vitaliy,
> 
> Thanks for looking to fix these warnings. Note that the compiler isn't
> always aware of some of the subtleties involved with things like
> cmpxchg. In cases where it thinks there may be an uninitialized usage,
> be sure to confirm it is possible before adding the overhead of an
> assignment to a hot path.
> 
> None of these assignments are necessary. Consider using __maybe_unused
> instead.

Darren,

Actually unused and uninitialized are different issues. __maybe_unused won't help in this case but there is another trick used in kernel for this:

#define uninitialized_var(x) x = x

So, here is updated patch.

>From 8eeaa5a97697bcc606aea23d32028aea7b271a96 Mon Sep 17 00:00:00 2001
From: Vitaliy Ivanov <vitalivanov@gmail.com>
Date: Thu, 7 Jul 2011 00:05:05 +0300
Subject: [PATCH] futex: uninitialized warning corrections
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kernel/futex.c: In function ‘fixup_pi_state_owner.clone.17’:
kernel/futex.c:1582:6: warning: ‘curval’ may be used uninitialized in this function
kernel/futex.c: In function ‘handle_futex_death’:
kernel/futex.c:2486:6: warning: ‘nval’ may be used uninitialized in this function
kernel/futex.c: In function ‘do_futex’:
kernel/futex.c:863:11: warning: ‘curval’ may be used uninitialized in this function
kernel/futex.c:828:6: note: ‘curval’ was declared here
kernel/futex.c:898:5: warning: ‘oldval’ may be used uninitialized in this function
kernel/futex.c:890:6: note: ‘oldval’ was declared here

Signed-off-by: Vitaliy Ivanov <vitalivanov@gmail.com>
---
 kernel/futex.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index fe28dc2..efb8e5b 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -825,7 +825,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
 {
 	struct task_struct *new_owner;
 	struct futex_pi_state *pi_state = this->pi_state;
-	u32 curval, newval;
+	u32 uninitialized_var(curval), newval;
 
 	if (!pi_state)
 		return -EINVAL;
@@ -887,7 +887,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
 
 static int unlock_futex_pi(u32 __user *uaddr, u32 uval)
 {
-	u32 oldval;
+	u32 uninitialized_var(oldval);
 
 	/*
 	 * There is no waiter, so we unlock the futex. The owner died
@@ -1546,7 +1546,7 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
 	u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
 	struct futex_pi_state *pi_state = q->pi_state;
 	struct task_struct *oldowner = pi_state->owner;
-	u32 uval, curval, newval;
+	u32 uval, uninitialized_var(curval), newval;
 	int ret;
 
 	/* Owner died? */
@@ -2451,7 +2451,7 @@ err_unlock:
  */
 int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
 {
-	u32 uval, nval, mval;
+	u32 uval, uninitialized_var(nval), mval;
 
 retry:
 	if (get_user(uval, uaddr))
-- 
1.7.4.1