linux-kernel - Re: [PATCH] printk: remove unneeded dead-store assignment

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Date:   Thu, 5 Nov 2020 20:49:08 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH] printk: remove unneeded dead-store assignment

On Fri, Nov 06, 2020 at 04:40:05AM +0100, Lukas Bulwahn wrote:
> make clang-analyzer on x86_64 defconfig caught my attention with:
> 
>   kernel/printk/printk_ringbuffer.c:885:3: warning:
>   Value stored to 'desc' is never read [clang-analyzer-deadcode.DeadStores]
>                 desc = to_desc(desc_ring, head_id);
>                 ^
> 
> Commit b6cf8b3f3312 ("printk: add lockless ringbuffer") introduced
> desc_reserve() with this unneeded dead-store assignment.
> 
> As discussed with John Ogness privately, this is probably just some minor
> left-over from previous iterations of the ringbuffer implementation. So,
> simply remove this unneeded dead assignment to make clang-analyzer happy.
> 
> 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.
> 
> Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
> applies cleanly on current master and next-20201105
> 
> John, please ack.
> Petr, please pick this minor non-urgent clean-up patch.
> 
>  kernel/printk/printk_ringbuffer.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> index 6b1525685277..74e25a1704f2 100644
> --- a/kernel/printk/printk_ringbuffer.c
> +++ b/kernel/printk/printk_ringbuffer.c
> @@ -882,8 +882,6 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
>  	head_id = atomic_long_read(&desc_ring->head_id); /* LMM(desc_reserve:A) */
>  
>  	do {
> -		desc = to_desc(desc_ring, head_id);
> -
>  		id = DESC_ID(head_id + 1);
>  		id_prev_wrap = DESC_ID_PREV_WRAP(desc_ring, id);
>  
> -- 
> 2.17.1
>