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

From: Ingo Molnar
Date: Fri May 01 2009 - 12:21:21 EST



* Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

>
> On Fri, 1 May 2009, Steven Rostedt wrote:
>
> >
> > On Fri, 1 May 2009, Ingo Molnar wrote:
> >
> > >
> > > * Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > >
> > > > From: Steven Rostedt <srostedt@xxxxxxxxxx>
> > > >
> > > > 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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