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

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.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.

	Ingo