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

* 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


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