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

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Message-ID: <alpine.DEB.2.00.0904302325200.20374@gandalf.stny.rr.com>
Date:	Thu, 30 Apr 2009 23:28:01 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 3/3] ring-buffer: make cpu buffer entries counter
 atomic




On Thu, 30 Apr 2009, Andrew Morton wrote:

> On Thu, 30 Apr 2009 22:22:13 -0400 Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > @@ -405,7 +405,7 @@ struct ring_buffer_per_cpu {
> >  	unsigned long			nmi_dropped;
> >  	unsigned long			commit_overrun;
> >  	unsigned long			overrun;
> > -	unsigned long			entries;
> > +	atomic_t			entries;
> 
> This switches `entries' from unsigned-64-bit to signed-32-bit.

Good point. Although I doubt we will have more that 2billion entries in 
one cpu buffer. But it can happen. I'll change that to 64 bit atomic.


> 
> The signedness thing is just an unfortunate thing with atomic_t and probably
> doesn't matter.
> 
> The change in size might or might not be a bug - that's for the person
> who didn't document the data structure to work out ;)
> 
> 
> 
> 
> /*
>  * head_page == tail_page && head == tail then buffer is empty.
>  */
> struct ring_buffer_per_cpu {
> 
> the comment refers to vaporfields?
> 

Ug, the head_page and tail_page are in this struct. But the "head" and 
"tail" are with in the "head_page" and "tail_page" respectively.

-- Steve



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