linux-kernel - Re: [PATCH 068/118] drbd: conn_printk() a dev

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Date:	Thu, 25 Aug 2011 11:16:25 -0700
From:	Joe Perches <joe@...ches.com>
To:	Philipp Reisner <philipp.reisner@...bit.com>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	drbd-dev@...ts.linbit.com
Subject: Re: [PATCH 068/118] drbd: conn_printk() a dev_printk() alike for
 drbd's connections

On Thu, 2011-08-25 at 17:08 +0200, Philipp Reisner wrote:
> Signed-off-by: Philipp Reisner <philipp.reisner@...bit.com>
> Signed-off-by: Lars Ellenberg <lars.ellenberg@...bit.com>
> ---
>  drivers/block/drbd/drbd_int.h  |    9 +++++++++
>  drivers/block/drbd/drbd_main.c |   16 +++++++++++++++-
>  2 files changed, 24 insertions(+), 1 deletions(-)

Couple of issues with this patch.

> diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
[]
> @@ -102,6 +102,15 @@ struct drbd_tconn;
[]
> +extern void conn_printk(const char *level, struct drbd_tconn *tconn, const char *fmt, ...);

This should be 

extern __attribute__((format (printf, 3, 4)))
void conn_printk(const char *level, struct drbd_tconn *tconn, const char  *fmt, ...);

to have gcc validate the printf format and arguments.

> diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
[]
> @@ -170,6 +170,18 @@ int _get_ldev_if_state(struct drbd_conf *mdev, enum drbd_disk_state mins)
>  
>  #endif
>  
> +/* printk functions for connections
> + */
> +void conn_printk(const char *level, struct drbd_tconn *tconn, const char *fmt, ...)
> +{
> +	va_list args;
> +
> +	printk("%sd-con %s: ", level, tconn->name);
> +	va_start(args, fmt);
> +	vprintk(fmt, args);
> +	va_end(args);

And using printk then vprintk is susceptible
to another thread interleaving a different
message between the printk and the vprintk.

Using struct va_format and %pV is better
because no message interleaving is possible.

ie:

void conn_printk(const char *level, struct drbd_tconn *tconn, const char *fmt, ...)
{
	struct va_format vaf;
	va_list args;

	va_start(args, fmt);

	vaf.fmt = fmt;
	vaf.va = &args;

	printk("%sd-con %s: %pV", level, tconn->name, &vaf);

	va_end(args);
}

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