linux-kernel - Re: [PATCH RESEND 2] taskstats: remove unneeded dead assignment

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Date:   Mon, 7 Mar 2022 15:30:57 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Balbir Singh <bsingharora@...il.com>, Tom Rix <trix@...hat.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 2] taskstats: remove unneeded dead assignment

On Mon,  7 Mar 2022 10:39:42 +0100 Lukas Bulwahn <lukas.bulwahn@...il.com> wrote:

> make clang-analyzer on x86_64 defconfig caught my attention with:
> 
>   kernel/taskstats.c:120:2: warning: Value stored to 'rc' is never read \
>   [clang-analyzer-deadcode.DeadStores]
>           rc = 0;
>           ^
> 
> Commit d94a041519f3 ("taskstats: free skb, avoid returns in
> send_cpu_listeners") made send_cpu_listeners() not return a value and
> hence, the rc variable remained only to be used within the loop where
> it is always assigned before read and it does not need any other
> initialisation.
> 
> So, simply remove this unneeded dead initializing assignment.
> 
> As compilers will detect this unneeded assignment and optimize this anyway,
> the resulting object code is identical before and after this change.
> 
> No functional change. No change to object code.
> 
> ...
>
> --- a/kernel/taskstats.c
> +++ b/kernel/taskstats.c
> @@ -117,7 +117,6 @@ static void send_cpu_listeners(struct sk_buff *skb,
>  
>  	genlmsg_end(skb, reply);
>  
> -	rc = 0;
>  	down_read(&listeners->sem);
>  	list_for_each_entry(s, &listeners->list, list) {
>  		skb_next = NULL;

Yup.  It would be better to also reduce the scope of `rc' so later code
can't go and read it uninitialized.

--- a/kernel/taskstats.c~taskstats-remove-unneeded-dead-assignment-fix
+++ a/kernel/taskstats.c
@@ -113,12 +113,14 @@ static void send_cpu_listeners(struct sk
 	struct listener *s, *tmp;
 	struct sk_buff *skb_next, *skb_cur = skb;
 	void *reply = genlmsg_data(genlhdr);
-	int rc, delcount = 0;
+	int delcount = 0;
 
 	genlmsg_end(skb, reply);
 
 	down_read(&listeners->sem);
 	list_for_each_entry(s, &listeners->list, list) {
+		int rc;
+
 		skb_next = NULL;
 		if (!list_is_last(&s->list, &listeners->list)) {
 			skb_next = skb_clone(skb_cur, GFP_KERNEL);
_

(we could just elimiate `rc' altogether, but I think the above is OK,
perhaps a little more readable).