linux-kernel - Re: kfifo has temporarily invalid in pointer?

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Date:	Mon, 15 Mar 2010 18:06:51 +0200
From:	Daniel Baluta <daniel.baluta@...il.com>
To:	"Robert P. J. Day" <rpjday@...shcourse.ca>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kfifo has temporarily invalid in pointer?

Hi Robert,

On Mon, Mar 15, 2010 at 4:58 PM, Robert P. J. Day <rpjday@...shcourse.ca> wrote:
>
>  (i am not trying to be annoyingly obsessive about the kernel kfifo,
> i am merely succeeding.)
:P
>  what appears to be a bit of an oddity WRT kfifo:  since a kfifo is
> defined with a fixed buffer size, it obviously enqueues and dequeues
> in a circular fashion.  so, the code to add some data to a kfifo (from
> kernel/kfifo.c):
>
> =====
> unsigned int kfifo_in(struct kfifo *fifo, const void *from,
>                                unsigned int len)
> {
>        len = min(kfifo_avail(fifo), len);
>
>        __kfifo_in_data(fifo, from, len, 0);
>        __kfifo_add_in(fifo, len);
>        return len;
> }
> =====
>
>  fair enough -- that first routine adds the data itself, while the
> second one correspondingly bumps up the pointer, which could
> conceivably wrap around to follow the data, correct?  but from
> include/linux.kfifo.h:len = min(kfifo_avail(fifo), len);

Wrong :). If you notice len is truncated using:
len = min(kfifo_avail(fifo), len);

>
> =====
> static inline void __kfifo_add_in(struct kfifo *fifo,
>                                unsigned int off)
> {
>        smp_wmb();
>        fifo->in += off;
> }
So,  fifo->in + min(kfifo_avail(fifo), len) < fifo->size, every time.

thanks,
Daniel.
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