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

    On Fri, 1 May 2009, Steven Rostedt wrote:

    >
    > On Fri, 1 May 2009, Ingo Molnar wrote:
    >
    > >
    > > * Steven Rostedt <rostedt@goodmis.org> wrote:
    > >
    > > > From: Steven Rostedt <srostedt@redhat.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".

    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.

    -- Steve



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