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

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Date:	Fri, 1 May 2009 12:31:58 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
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


On Fri, 1 May 2009, Ingo Molnar wrote:
> > > > > the counter too. This would cause missing entries to be added.
> > > > 
> > > > > -	unsigned long			entries;
> > > > > +	atomic_t			entries;
> > > > 
> > > > Hm, that's not really good as atomics can be rather expensive and 
> > > > this is the fastpath.
> > > 
> > > Actually, it could be local_t. I used that in a lot of the other places.
> > > The race is with on CPU not other CPUs, and on archs like x86 there
> > > is not cost of the "LOCK".
> > 
> > Ug, it must be atomic_t. It is also modified by the reader. Thus 
> > it is not only a race with a single CPU but also multiple CPUs.
> > 
> > This means that interrupts disabled is not the only proctection it 
> > needs. It must either be an atomic, or protected by a spinlock.
> 
> Trace buffers are rather fundamentally per cpu. Where's the problem?

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.

-- Steve

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