linux-kernel - Re: [PATCH 3/3] ring-buffer: make cpu buffer entries counter atomic

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Message-ID: <20090501174228.GB9565@elte.hu>
Date:	Fri, 1 May 2009 19:42:28 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 3/3] ring-buffer: make cpu buffer entries counter atomic


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> On Fri, 1 May 2009, Ingo Molnar wrote:
> 
> > 
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > On Fri, 1 May 2009, Ingo Molnar wrote:
> > > > > The entries keeps track of the number of entries in the buffer. A 
> > > > > writer (producer) adds to the counter and readers (consumers) 
> > > > > subtract from them. A writer can subtract them if it overwrites a 
> > > > > page before the producer consumes it.
> > > > > 
> > > > > Only the writers are pinned to a CPU, the readers happen on any 
> > > > > CPU.
> > > > 
> > > > But that does not require atomicity. It requires careful use of 
> > > > barriers, but otherwise atomicity is not needed. Update of machine 
> > > > word variables (if they are aligned to a machine word) is guaranteed 
> > > > to be atomic, even without atomic_t overhead.
> > > 
> > > I'm confused :-/ This throws out all that I learned in multi threaded 
> > > programming.
> > > 
> > > If I have a shared variable used by two threads, the adding and 
> > > subtracting of that variable does not need to be atomic?
> > > 
> > >         CPU0                 CPU1
> > >         ----                 ----
> > > 	load A               load A
> > > 	sub  1, A            sub 1, A
> > > 	store A              store A
> > > 
> > > can work??
> > 
> > no, that wont work. But as long as there's just a single CPU that is 
> > a _writer_ (does stores), it can be observed in an atomic/coherent 
> > manner, without the use of atomics.
> 
> Ah, maybe there's confusion in my explanation. When I talk about 
> writers and readers, I'm talking about those writers into the ring 
> buffer and readers from the ring buffer. But both writers and 
> readers write to the entries counter. Readers subtract and writers 
> add. But writers can also subtract on overruns.

a solution for that would be to split it into two counts - for both 
sides. Or to eliminate it if possible. We _really_ need to make the 
ring-buffer _much_ cheaper than it is today.
y
	Ingo
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