linux-kernel - Re: [PATCH v2] printk: ringbuffer: Improve prb_next

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Message-ID: <875ytih3j9.fsf@jogness.linutronix.de>
Date:   Wed, 27 Oct 2021 17:54:42 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     "chunlei.wang" <Chunlei.wang@...iatek.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] printk: ringbuffer: Improve prb_next_seq() performance

On 2021-10-27, Petr Mladek <pmladek@...e.com> wrote:
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> index 8a7b7362c0dd..24f47fbefbb5 100644
> --- a/kernel/printk/printk_ringbuffer.c
> +++ b/kernel/printk/printk_ringbuffer.c
> @@ -474,8 +474,10 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
>  	 * state has been re-checked. A memcpy() for all of @desc
>  	 * cannot be used because of the atomic_t @state_var field.
>  	 */
> -	memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
> -	       sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
> +	if (desc_out) {
> +		memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
> +		       sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
> +	}
>  	if (seq_out)
>  		*seq_out = info->seq; /* also part of desc_read:C */
>  	if (caller_id_out)

You also need this hunk:

@@ -530,7 +530,8 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
 	state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
 	d_state = get_desc_state(id, state_val);
 out:
-	atomic_long_set(&desc_out->state_var, state_val);
+	if (desc_out)
+		atomic_long_set(&desc_out->state_var, state_val);
 	return d_state;
 }

Sorry, I missed it in my last review. This time I build+boot tested the patch.

With this hunk added:

Reviewed-by: John Ogness <john.ogness@...utronix.de>

John