linux-kernel - Re: [PATCH] relay: Add global mode support for buffer-only channels

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Date:	Mon, 4 Jul 2016 09:00:38 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	akash.goel@...el.com
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH] relay: Add global mode support for buffer-only channels

On Sun, Jul 03, 2016 at 09:45:27PM +0530, akash.goel@...el.com wrote:
> From: Akash Goel <akash.goel@...el.com>
> 
> The following patch added support to use channels with no associated files.
> 	relay: add buffer-only channels; useful for early logging
> This is useful when the exact location of relay file is not known or the
> the parent directory of relay file is not available, while creating the
> channel and the logging has to start right from the boot.
> 
> But there was no provision to use global mode with buffer-only channels,
> which is added by this patch, without modifying the interface where initially
> there will be a dummy invocation of create_buf_file callback through which
> kernel client can convey the need of a global buffer.
> 
> For the use case where drivers/kernel clients want a simple interface for the
> userspace, which enables them to capture data/logs from relay file in order &
> without any post processing, support of Global buffer mode is warranted.
> 
> Cc: Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
> Cc: Tom Zanussi <tzanussi@...il.com>
> Cc: Chris Wilson <chris@...is-wilson.co.uk>
> Signed-off-by: Akash Goel <akash.goel@...el.com>
> ---
>  kernel/relay.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/kernel/relay.c b/kernel/relay.c
> index 04d7cf3..b267384 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -451,6 +451,16 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
>  		if (!dentry)
>  			goto free_buf;
>  		relay_set_buf_dentry(buf, dentry);
> +	} else {

The trailing block can be replaced with:

		/* Only retrieve global info, nothing more, nothing less */

> +		dentry = chan->cb->create_buf_file(NULL, NULL,
> +					S_IRUSR, buf,
> +					&chan->is_global);

Watch alignment, align to the opening bracket.

> +		if (WARN_ON(dentry))
> +			goto free_buf;
>  	}
>  
>   	buf->cpu = cpu;
> @@ -666,6 +676,27 @@ int relay_late_setup_files(struct rchan *chan,
>  	}
>  	chan->has_base_filename = 1;
>  	chan->parent = parent;
> +
> +	if (chan->is_global) {
> +		if (unlikely(!chan->buf[0])) {

if (WARN_ON_ONCE(!chan->buf[0])) {

> +			WARN_ONCE(1, KERN_ERR "CPU 0 has no buffer!\n");

The WARN includes the unlikely and the message here isn't any more
informative than the line itself, plus has the erroneous KERN_ERR

> +			mutex_unlock(&relay_channels_mutex);
> +			return -EINVAL;
> +                }
> +
> +		dentry = relay_create_buf_file(chan, chan->buf[0], 0);
> +
> +		if (unlikely(!dentry))
> +			err = -EINVAL;
> +		else if (WARN_ON(!chan->is_global))

?

> +			err = -EINVAL;
> +		else
> +			relay_set_buf_dentry(chan->buf[0], dentry);
> +
> +		mutex_unlock(&relay_channels_mutex);
> +		return err;

This could be written:

if (chan->is_global) {
	err = -EINVAL;
	if (!WARN_ON_ONCE(!chan->buf[0])) {
		dentry = relay_create_buf_file(chan, chan->buf[0], 0))
		if (dentry) {
			relay_set_buf_dentry(chan->buf[0], dentry);
			err = 0;
		}
	}
	mutex_unlock();
	return err;
}

for a single exit/unlock path in this block.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre