linux-kernel - Re: [PATCH] kfifo: fix warn_unused

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Message-Id: <20091213142907.d0ac63da.akpm@linux-foundation.org>
Date:	Sun, 13 Dec 2009 14:29:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Andi Kleen <andi@...stfloor.org>,
	Amerigo Wang <xiyou.wangcong@...il.com>,
	Joe Perches <joe@...ches.com>,
	Roger Quadros <quadros.roger@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Mauro Carvalho Chehab <mchehab@...hat.com>
Subject: Re: [PATCH] kfifo: fix warn_unused_result

On Fri, 11 Dec 2009 10:18:28 +0100 Stefani Seibold <stefani@...bold.net> wrote:

> As requested by Andrew Morton:
> 
> This patch fix the "ignoring return value of '...', declared with
> attribute warn_unused_result" compiler warning in several users of the
> new kfifo API.
> 
> The patch-set is against current mm tree from 11-Dec-2009
> 
> ...
>
> --- mmotm/drivers/char/nozomi.c	2009-12-11 08:31:46.670736197 +0100
> +++ linux-2.6.32/drivers/char/nozomi.c	2009-12-11 09:25:46.941436203 +0100
> @@ -685,8 +685,9 @@ static int nozomi_read_config_table(stru
>  		dump_table(dc);
>  
>  		for (i = PORT_MDM; i < MAX_PORT; i++) {
> -			kfifo_alloc(&dc->port[i].fifo_ul,
> -				FIFO_BUFFER_SIZE_UL, GFP_ATOMIC);
> +			if (kfifo_alloc(&dc->port[i].fifo_ul,
> +				FIFO_BUFFER_SIZE_UL, GFP_ATOMIC))
> +					BUG();

No, we can't do this.  GFP_ATOMIC allocations are unreliable and can
fail.  The calling code *has* to detect the failure and then take some
recovery action.

It would be better to leave the warning in place, rather than to add
this runtime landmine.

>  		input_sync(kp.dev);
> -		kfifo_in_locked(&sonypi_device.input_fifo,
> +		if (kfifo_in_locked(&sonypi_device.input_fifo,
>  			(unsigned char *)&kp, sizeof(kp),
> -			&sonypi_device.input_fifo_lock);
> +			&sonypi_device.input_fifo_lock) != sizeof(kp))
> +				BUG();

The rest of the patch seems to be adding BUG()s if kfifo_in() fails. 
All over the place.

If that's the appropriate way to handle failure for these callsites
then it would be neater to do this in the callee.  ie, add a new

unsigned int kfifo_in_nonpartial(struct kfifo *fifo,
                        const unsigned char *from, unsigned int len)
{
	unsigned int ret = kfifo_in(fifo, from, len);

	BUG_ON(ret != len);
	return ret;
}

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