On Fri 14-11-14 13:19:31, Alex Elder 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
> needed, neither was it required to use ACCESS_ONCE(). I
> offered an elaborate description that concluded Stephen
> 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 value (and not an out-of-date cached one).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.pr...@gmail.com>
> Fix-suggested-by: Steven Rostedt <rost...@goodmis.org>
> Signed-off-by: Alex Elder <el...@linaro.org>
> ---
> Note!!! I am not able to stress test this patch.
The patch looks good to me. You can add:
Reviewed-by: Jan Kara <j...@suse.cz>
BTW, stress-testing this is tough. I never saw printk recursion happening
in practice - only when I screwed up something in printk code when
experimenting.
Honza
> 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;
> --
> 1.9.1
>
--
Jan Kara <j...@suse.cz>
SUSE Labs, CR
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
