linux-kernel - Re: kfifo: possible weird violation of what should be invariant

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Date:	Tue, 16 Mar 2010 15:25:17 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	"Robert P. J. Day" <rpjday@...shcourse.ca>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kfifo:  possible weird violation of what should be invariant

 >   sure, the code seems to work, but allowing the internal values of a
 > kfifo to contain invalid values on a regular basis would seem to make
 > a mess of, say, tracing or debugging.  making sure that offset values
 > actually lie within their valid range would seem to be one of those
 > ASSERT() things that should always be true, should it not?  is there a
 > reason the design is like this?

Actually I believe having the values be free-running without clamping
them makes the code much simpler -- the reason being that you preserve
the invariant of "in" always being ahead of "out".  If you reduce the
pointers modulo the size, then you end up having a lot of code that has
two cases: one to handle "in > out", and one to handle "in < out because
in has wrapped and out hasn't yet".

 - R.
-- 
Roland Dreier  <rolandd@...co.com>
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