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    SubjectRe: [PATCH] printk: drop logbuf_cpu volatile qualifier
    On Fri, 14 Nov 2014 13:19:31 -0600
    Alex Elder <elder@linaro.org> wrote:

    > Pranith Kumar posted a patch in which removed the "volatile"
    > qualifier for the "logbuf_cpu" variable in vprintk_emit().
    > https://lkml.org/lkml/2014/11/13/894
    > In his patch, he used ACCESS_ONCE() for all references to
    > that symbol to provide whatever protection was intended.
    >
    > There was some discussion that followed, and in the end
    > Stephen Rostedt concluded that not only was "volatile" not

    I don't know of a Stephen Rostedt. J

    > needed, neither was it required to use ACCESS_ONCE(). I
    > offered an elaborate description that concluded Stephen

    Who is this Stephen you talk about?

    > was right, and Pranith asked me to submit an alternative
    > patch. And this is it.
    >
    > The basic reason "volatile" is not needed is that "logbuf_cpu" has
    > static storage duration, and vprintk_emit() is an exported
    > interface. This means that the value of logbuf_cpu must be read
    > from memory the first time it is used in a particular call of
    > vprintk_emit(). The variable's value is read only once in that
    > function, when it's read it'll be the copy from memory (or cache).
    >
    > In addition, the value of "logbuf_cpu" is only ever written under
    > protection of a spinlock. So the value that is read is the "real"
    > value (and not an out-of-date cached one). If its value is not
    > UINT_MAX, it is the current CPU's processor id, and it will have
    > been last written by the running CPU.
    >
    > Reported-by: Pranith Kumar <bobby.prani@gmail.com>
    > Fix-suggested-by: Steven Rostedt <rostedt@goodmis.org>

    Ah, I know of him!


    > Signed-off-by: Alex Elder <elder@linaro.org>
    > ---
    > Note!!! I am not able to stress test this patch.

    I know the guys at SuSE are stressing printk a bit. Maybe they can come
    up with something. (Cc'd).

    Thanks,

    -- Steve

    >
    > kernel/printk/printk.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    > index ced2b84..fefc8d0 100644
    > --- a/kernel/printk/printk.c
    > +++ b/kernel/printk/printk.c
    > @@ -1627,7 +1627,7 @@ asmlinkage int vprintk_emit(int facility, int level,
    > int printed_len = 0;
    > bool in_sched = false;
    > /* cpu currently holding logbuf_lock in this function */
    > - static volatile unsigned int logbuf_cpu = UINT_MAX;
    > + static unsigned int logbuf_cpu = UINT_MAX;
    >
    > if (level == SCHED_MESSAGE_LOGLEVEL) {
    > level = -1;



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