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

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Date:	Fri, 1 May 2009 18:20:53 +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, Steven Rostedt wrote:
> 
> > 
> > On Fri, 1 May 2009, Ingo Molnar wrote:
> > 
> > > 
> > > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > > 
> > > > From: Steven Rostedt <srostedt@...hat.com>
> > > > 
> > > > The entries counter in cpu buffer is not atomic. Although it only 
> > > > gets updated by a single CPU, interrupts may come in and update 
> > > > 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?

	Ingo
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