From: Duncan Sands <baldrick@free.fr>
To: Stelian Pop <stelian@popies.net>
Subject: Re: [RFC, 2.6] a simple FIFO implementation
Date: Fri, 17 Sep 2004 15:00:35 +0200
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On Friday 17 September 2004 14:48, Stelian Pop wrote:
> On Fri, Sep 17, 2004 at 02:37:57PM +0200, Duncan Sands wrote:
> 
> > > + * Note that with only one concurrent reader and one concurrent 
> > > + * writer, you don't need extra locking to use these functions.
> >                                  ^^^^^ which functions? (ambiguous)
> 
> Well, the same comment is for two adjacent functions, so I don't
> think it's so ambiguous. Or s/these/this/ if you prefer.
> 
> > And what does "extra locking" mean?
> 
> Some kind of locking, like the one the wrapper kfifo_get/kfifo_put
> propose.
> 
> > > +	len = min(len, fifo->size - fifo->in + fifo->out);
> > 
> > After all, since you are reading both in and out here, some kind of
> > locking is needed.
> 
> But the order in which in and out get modified guarantees that you
> will still have a coherent content (provided the assignments are 
> atomic, which they are).

Hi Stelian, what is to stop the compiler putting, say, "in" in a register
for the process calling __kfifo_get, so that it only sees a constant
value.  Then after a while that process will think there is nothing
to get even though another process is shoving stuff into the fifo and
modifying "in".

By the way, the comment for __kfifo_get has a typo:

+ * kfifo_get - gets some data from the FIFO, no locking version
    ^^^^^^^^^ should be __kfifo_get

Ciao,

Duncan.
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