Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

John Ogness
2019-06-24T13:55:44Z

Hi,

I would like to point out an important typo related to memory
barriers...

On 2019-06-07, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/lib/printk_ringbuffer.c b/lib/printk_ringbuffer.c
> new file mode 100644
> index 000000000000..d0b2b6a549b0
> --- /dev/null
> +++ b/lib/printk_ringbuffer.c
[...]
> +static struct prb_descr *remove_oldest_descr(struct printk_ringbuffer *rb)
> +{
> + struct prb_list *l = &rb->descr_list;
> + unsigned long oldest_id;
> + struct prb_descr *d;
> + unsigned long next;
> +
> + for (;;) {
> + oldest_id = READ_ONCE(l->oldest);
> +
> + /* list empty */
> + if (oldest_id == EOL)
> + return NULL;
> +
> + d = TO_DESCR(rb, oldest_id);
> +
> + /* only descriptors with _invalid_ data can be removed */
> + if (data_valid(rb, READ_ONCE(rb->data_list.oldest),
> + READ_ONCE(rb->data_list.newest),
> + READ_ONCE(d->data),
> + READ_ONCE(d->data_next))) {
> + return NULL;
> + }
> +
> + /*
> + * MB6: synchronize link descr
> + *
> + * In particular: l->oldest is loaded as a data dependency so
> + * d->next and the following l->oldest will load afterwards,
> + * respectively.
> + */
> + next = smp_load_acquire(&d->next);
> +
> + if (next == EOL && READ_ONCE(l->oldest) == oldest_id) {
> + /*
> + * The oldest has no next, so this is a list of one
> + * descriptor. Lists must always have at least one
> + * descriptor.
> + */
> + return NULL;
> + }
> +
> + if (cmpxchg(&l->oldest, oldest_id, next) == oldest_id) {
> + /* removed successfully */
> + break;
> + }

This is supposed to be cmpxchg_relaxed(), not cmpxchg(). I did not
intend to include the general mb() memory barriers around the RMW
operation. (For some reason I thought _relaxed was the default.) Sorry.

John Ogness