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

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Date:	Fri, 1 May 2009 10:23:40 -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:

> 
> * 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".

> 
> This is the upteenth time or so that the fact that we do not disable 
> irqs while generating trace entries bites us in one way or another. 
> IRQs can come in and confuse function trace output, etc. etc.

Note, this race is on a simple counter used for stats. It never was
exposed to user land except in the latency output, and that tracer 
disables interrupts anyway.

> 
> Please lets do what i suggested a long time ago: disable irqs _once_ 
> in any trace point and run atomically from that point on, and enable 
> them once, at the end.
> 
> The cost is very small and it turns into a win immediately by 
> elimination of a _single_ atomic instruction. (even on Nehalem they 
> cost 20 cycles. More on older CPUs.) We can drop the preempt-count 
> disable/enable as well and a lot of racy code as well. Please.

If we punt and simply disable interrupts in the ring buffer, I would then 
have to disable all tracing of NMIs. Yes it will make the code simpler, 
but the new code would also have:

ring_buffer_lock_reserve() {

	if (in_nmi())
		return NULL;

If that is acceptible, then fine. I'll make the change.

I will also throw away the lockless ring buffer since it would no long er 
be needed.

-- Steve

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