dataring_push() barriers Re: [RFC PATCH v4 1/9] printk-rb: add a new printk ringbuffer implementation

Petr Mladek
2019-08-20T13:50:08Z

On Thu 2019-08-08 00:32:26, John Ogness wrote:
> +/**
> + * dataring_push() - Reserve a data block in the data array.
> + *
> + * @dr: The data ringbuffer to reserve data in.
> + *
> + * @size: The size to reserve.
> + *
> + * @desc: A pointer to a descriptor to store the data block information.
> + *
> + * @id: The ID of the descriptor to be associated.
> + * The data block will not be set with @id, but rather initialized with
> + * a value that is explicitly different than @id. This is to handle the
> + * case when newly available garbage by chance matches the descriptor
> + * ID.
> + *
> + * This function expects to move the head pointer forward. If this would
> + * result in overtaking the data array index of the tail, the tail data block
> + * will be invalidated.
> + *
> + * Return: A pointer to the reserved writer data, otherwise NULL.
> + *
> + * This will only fail if it was not possible to invalidate the tail data
> + * block.
> + */
> +char *dataring_push(struct dataring *dr, unsigned int size,
> + struct dr_desc *desc, unsigned long id)
> +{
> + unsigned long begin_lpos;
> + unsigned long next_lpos;
> + struct dr_datablock *db;
> + bool ret;
> +
> + to_db_size(&size);
> +
> + do {
> + /* fA: */
> + ret = get_new_lpos(dr, size, &begin_lpos, &next_lpos);
> +
> + /*
> + * fB:
> + *
> + * The data ringbuffer tail may have been pushed (by this or
> + * any other task). The updated @tail_lpos must be visible to
> + * all observers before changes to @begin_lpos, @next_lpos, or
> + * @head_lpos by this task are visible in order to allow other
> + * tasks to recognize the invalidation of the data
> + * blocks.

This sounds strange. The write barrier should be done only on CPU
that really modified tail_lpos. I.e. it should be in _dataring_pop()
after successful dr->tail_lpos modification.

> + * This pairs with the smp_rmb() in _dataring_pop() as well as
> + * any reader task using smp_rmb() to post-validate data that
> + * has been read from a data block.
> +
> + * Memory barrier involvement:
> + *
> + * If dE reads from fE, then dI reads from fA->eA.
> + * If dC reads from fG, then dI reads from fA->eA.
> + * If dD reads from fH, then dI reads from fA->eA.
> + * If mC reads from fH, then mF reads from fA->eA.
> + *
> + * Relies on:
> + *
> + * FULL MB between fA->eA and fE
> + * matching
> + * RMB between dE and dI
> + *
> + * FULL MB between fA->eA and fG
> + * matching
> + * RMB between dC and dI
> + *
> + * FULL MB between fA->eA and fH
> + * matching
> + * RMB between dD and dI
> + *
> + * FULL MB between fA->eA and fH
> + * matching
> + * RMB between mC and mF
> + */
> + smp_mb();

All these comments talk about sychronization against read barriers.
It means that we would need a write barrier here. But it does
not make much sense to do write barrier before actually
writing dr->head_lpos.

After all I think that we do not need any barrier here.
The write barrier for dr->tail_lpos should be in
_dataring_pop(). The read barrier is not needed because
we are not reading anything here.

Instead we should put a barrier after modyfying dr->head_lpos,
see below.

> + if (!ret) {
> + /*
> + * Force @desc permanently invalid to minimize risk
> + * of the descriptor later unexpectedly being
> + * determined as valid due to overflowing/wrapping of
> + * @head_lpos. An unaligned @begin_lpos can never
> + * point to a data block and having the same value
> + * for @begin_lpos and @next_lpos is also invalid.
> + */
> +
> + /* fC: */
> + WRITE_ONCE(desc->begin_lpos, 1);
> +
> + /* fD: */
> + WRITE_ONCE(desc->next_lpos, 1);
> +
> + return NULL;
> + }
> + /* fE: */
> + } while (atomic_long_cmpxchg_relaxed(&dr->head_lpos, begin_lpos,
> + next_lpos) != begin_lpos);
> +

We need a write barrier here to make sure that dr->head_lpos
is updated before we start updating other values, e.g.
db->id below.

Best Regards,
Petr

> + db = to_datablock(dr, begin_lpos);
> +
> + /*
> + * fF:
> + *
> + * @db->id is a garbage value and could possibly match the @id. This
> + * would be a problem because the data block would be considered
> + * valid before the writer has finished with it (i.e. before the
> + * writer has set @id). Force some other ID value.
> + */
> + WRITE_ONCE(db->id, id - 1);
>
> + /*
> + * fG:
> + *
> + * Ensure that @db->id is initialized to a wrong ID value before
> + * setting @begin_lpos so that there is no risk of accidentally
> + * matching a data block to a descriptor before the writer is finished
> + * with it (i.e. before the writer has set the correct @id). This
> + * pairs with the _acquire() in _dataring_pop().
> + *
> + * Memory barrier involvement:
> + *
> + * If dC reads from fG, then dF reads from fF.
> + *
> + * Relies on:
> + *
> + * RELEASE from fF to fG
> + * matching
> + * ACQUIRE from dC to dF
> + */
> + smp_store_release(&desc->begin_lpos, begin_lpos);
> +
> + /* fH: */
> + WRITE_ONCE(desc->next_lpos, next_lpos);
> +
> + /* If this data block wraps, use @data from the content data block. */
> + if (DATA_WRAPS(dr, begin_lpos) != DATA_WRAPS(dr, next_lpos))
> + db = to_datablock(dr, 0);
> +
> + return &db->data[0];
> +}