linux-kernel - Re: [PATCH 3/4] ring-buffer: add ring_buffer_discard

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Date:	Wed, 1 Apr 2009 22:48:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 3/4] ring-buffer: add ring_buffer_discard_commit

On Thu, 02 Apr 2009 01:27:24 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <srostedt@...hat.com>
> 
> The ring_buffer_discard_commit is similar to ring_buffer_event_discard
> but it can only be done on an event that has yet to be commited.
> Unpredictable results can happen otherwise.
> 
> The main difference between ring_buffer_discard_commit and
> ring_buffer_event_discard is that ring_buffer_discard_commit will try
> to free the data in the ring buffer if nothing has addded data
> after the reserved event. If something did, then it acts almost the
> same as ring_buffer_event_discard followed by a
> ring_buffer_unlock_commit.
> 
> Note, either ring_buffer_commit_discard and ring_buffer_unlock_commit
> can be called on an event, not both.
> 
> This commit also exports both discard functions to be usable by
> GPL modules.
> 
> ...
>
> +/*
> + * ring_buffer_event_discard can discard any event in the ring buffer.
> + *   it is up to the caller to protect against a reader from
> + *   consuming it or a writer from wrapping and replacing it.
> + *
> + * No external protection is needed if this is called before
> + * the event is commited. But in that case it would be better to
> + * use ring_buffer_discard_commit.
> + *
> + * Note, if an event that has not been committed is discarded
> + * with ring_buffer_event_discard, it must still be committed.
> + */
>  void ring_buffer_event_discard(struct ring_buffer_event *event);
>  
> ...
>
>  /**
> + * ring_buffer_event_discard - discard any event in the ring buffer
> + * @event: the event to discard
> + *
> + * Sometimes a event that is in the ring buffer needs to be ignored.
> + * This function lets the user discard an event in the ring buffer
> + * and then that event will not be read later.
> + *
> + * Note, it is up to the user to be careful with this, and protect
> + * against races. If the user discards an event that has been consumed
> + * it is possible that it could corrupt the ring buffer.
> + */
> +void ring_buffer_event_discard(struct ring_buffer_event *event)
> +{
> +	event->type = RINGBUF_TYPE_PADDING;
> +	/* time delta must be non zero */
> +	if (!event->time_delta)
> +		event->time_delta = 1;
> +}
> +EXPORT_SYMBOL_GPL(ring_buffer_event_discard);

Seems a bit cumbersome to document the interface partly in the .h file
and partly in the .c file?

Doing all of it in the .c file would be typical.
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