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SubjectRe: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer)
Hi John,

Sorry for the delay.

I don't have an overall understanding of the patch(-set) yet, so I limit
to a couple of general questions about the memory barriers introduced by
the path. Please see inline comments.


> + *desc_out = READ_ONCE(*desc);
> +
> + /* Load data before re-checking state. */
> + smp_rmb(); /* matches LMM_REF(desc_reserve:A) */

I looked for a matching WRITE_ONCE() or some other type of marked write,
but I could not find it. What is the rationale? Or what did I miss?


> + do {
> + next_lpos = get_next_lpos(data_ring, begin_lpos, size);
> +
> + if (!data_push_tail(rb, data_ring,
> + next_lpos - DATA_SIZE(data_ring))) {
> + /* Failed to allocate, specify a data-less block. */
> + blk_lpos->begin = INVALID_LPOS;
> + blk_lpos->next = INVALID_LPOS;
> + return NULL;
> + }
> + } while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
> + next_lpos));
> +
> + /*
> + * No barrier is needed here. The data validity is defined by
> + * the state of the associated descriptor. They are marked as
> + * invalid at the moment. And only the winner of the above
> + * cmpxchg() could write here.
> + */

The (successful) CMPXCHG provides a full barrier. This comment suggests
that that could be somehow relaxed? Or the comment could be improved?

(The patch introduces a number of CMPXCHG: similar questions would apply
to those other instances...)

Thanks,
Andrea

P. S. Please use my @gmail.com address for future communications.

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