linux-kernel - Re: [PATCH] Document Linux's circular buffering capabilities

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Date:	Sat, 13 Mar 2010 10:26:15 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	David Howells <dhowells@...hat.com>
CC:	torvalds@...l.org, akpm@...ux-foundation.org, sgruszka@...hat.com,
	davem@...emloft.net, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Randy Dunlap <rdunlap@...otime.net>
Subject: Re: [PATCH] Document Linux's circular buffering capabilities

David Howells wrote:
> Stefan Richter <stefanr@...6.in-berlin.de> wrote:
>> "...do this" could be misunderstood as "use a modulus instruction",
...
> How about:
> 
> 	Calculation of the occupancy or the remaining capacity of an
> 	arbitrarily sized circular buffer would normally be a slow operation,
> 	requiring the use of a modulus (divide) instruction.  However, if the
> 	buffer is of a power-of-2 size,
[...]

Yep.

>> I don't understand why ACCESS_ONCE is needed here.  The CIRC_SPACE and
>> CIRC_CNT macros do not look at head and tail more than once.
> 
> In this example they don't, but say someone wants to read several elements
> from the buffer, they might end up accessing their copy of head several times.

Would you agree to add a quick note that these examples are simple
enough to not strictly require ACCESS_ONCE but are meant to show what
more general code would have to do?  Else a reader might be left puzzled
why he can't see in the example code the circumstances which require
ACCESS_ONCE and may remain unsure about where to use it in his own works...

(BTW, good that I came across your documentation posting.  This twist
with possibly multiple loads was not apparent to me too.  I am going to
have to have another look at some driver code with this in mind...)
-- 
Stefan Richter
-=====-==-=- --== -==-=
http://arcgraph.de/sr/
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