linux-kernel - Re: [PATCH v5 2/4] printk: add lockless ringbuffer

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Date:   Fri, 10 Jul 2020 17:43:05 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/4] printk: add lockless ringbuffer

On (20/07/09 15:29), John Ogness wrote:
[..]
> +/*
> + * A data block: mapped directly to the beginning of the data block area
> + * specified as a logical position within the data ring.
> + *
> + * @id:   the ID of the associated descriptor
> + * @data: the writer data
> + *
> + * Note that the size of a data block is only known by its associated
> + * descriptor.
> + */
> +struct prb_data_block {
> +	unsigned long	id;
> +	char		data[0];
> +};

A nit: I think someone will send "Replace zero-length arrays with
flexible array member" soon enough:

-	char		data[0];
+	char		data[];

[..]
> +/*
> + * Sanity checker for reserve size. The ringbuffer code assumes that a data
> + * block does not exceed the maximum possible size that could fit within the
> + * ringbuffer. This function provides that basic size check so that the
> + * assumption is safe.
> + *
> + * Writers are also not allowed to write 0-sized (data-less) records. Such
> + * records are used only internally by the ringbuffer.
> + */
> +static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
> +{
> +	struct prb_data_block *db = NULL;
> +
> +	/*
> +	 * Writers are not allowed to write data-less records. Such records
> +	 * are used only internally by the ringbuffer to denote records where
> +	 * their data failed to allocate or have been lost.
> +	 */

A nit: The same data-less records comment is some 5 lines earlier. But OK.

> +	if (size == 0)
> +		return false;

[..]

> +void prb_init(struct printk_ringbuffer *rb,
> +	      char *text_buf, unsigned int textbits,
> +	      char *dict_buf, unsigned int dictbits,
> +	      struct prb_desc *descs, unsigned int descbits)
> +{
> +	memset(descs, 0, _DESCS_COUNT(descbits) * sizeof(descs[0]));
> +
> +	rb->desc_ring.count_bits = descbits;
> +	rb->desc_ring.descs = descs;
> +	atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
> +	atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
> +
> +	rb->text_data_ring.size_bits = textbits;
> +	rb->text_data_ring.data = text_buf;
> +	atomic_long_set(&rb->text_data_ring.head_lpos, BLK0_LPOS(textbits));
> +	atomic_long_set(&rb->text_data_ring.tail_lpos, BLK0_LPOS(textbits));
> +
> +	rb->dict_data_ring.size_bits = dictbits;
> +	rb->dict_data_ring.data = dict_buf;
> +	atomic_long_set(&rb->dict_data_ring.head_lpos, BLK0_LPOS(dictbits));
> +	atomic_long_set(&rb->dict_data_ring.tail_lpos, BLK0_LPOS(dictbits));
> +

Just a side note: some people want !CONFIG_PRINTK builds. I wonder
how many people will want !CONFIG_PRINTK_DICT. The core logbuf/dict
logbuf split is really cool.

	-ss