Full barrier in data_push_tail(): was [PATCH v2 2/3] printk: add lockless buffer

[Petr Mladek]

On Fri 2020-05-01 11:46:09, John Ogness wrote:
> Introduce a multi-reader multi-writer lockless ringbuffer for storing
> the kernel log messages. Readers and writers may use their API from
> any context (including scheduler and NMI). This ringbuffer will make
> it possible to decouple printk() callers from any context, locking,
> or console constraints. It also makes it possible for readers to have
> full access to the ringbuffer contents at any time and context (for
> example from any panic situation).
> 
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> new file mode 100644
> index 000000000000..e0a66468d4f3
> --- /dev/null
> +++ b/kernel/printk/printk_ringbuffer.c
> +/*
> + * Advance the data ring tail to at least @lpos. This function puts
> + * descriptors into the reusable state if the tail is pushed beyond
> + * their associated data block.
> + */
> +static bool data_push_tail(struct printk_ringbuffer *rb,
> +			   struct prb_data_ring *data_ring,
> +			   unsigned long lpos)
> +{
> +	unsigned long tail_lpos;
> +	unsigned long next_lpos;
> +
> +	/* If @lpos is not valid, there is nothing to do. */
> +	if (lpos == INVALID_LPOS)
> +		return true;
> +
> +	tail_lpos = atomic_long_read(&data_ring->tail_lpos);
> +
> +	do {
> +		/* Done, if the tail lpos is already at or beyond @lpos. */
> +		if ((lpos - tail_lpos) - 1 >= DATA_SIZE(data_ring))
> +			break;
> +
> +		/*
> +		 * Make all descriptors reusable that are associated with
> +		 * data blocks before @lpos.
> +		 */
> +		if (!data_make_reusable(rb, data_ring, tail_lpos, lpos,
> +					&next_lpos)) {
> +			/*
> +			 * Guarantee the descriptor state loaded in
> +			 * data_make_reusable() is performed before reloading
> +			 * the tail lpos. The failed data_make_reusable() may
> +			 * be due to a newly recycled descriptor causing
> +			 * the tail lpos to have been previously pushed. This
> +			 * pairs with desc_reserve:D.
> +			 *
> +			 * Memory barrier involvement:
> +			 *
> +			 * If data_make_reusable:D reads from desc_reserve:G,
> +			 * then data_push_tail:B reads from data_push_tail:D.
> +			 *
> +			 * Relies on:
> +			 *
> +			 * MB from data_push_tail:D to desc_reserve:G
> +			 *    matching
> +			 * RMB from data_make_reusable:D to data_push_tail:B
> +			 *
> +			 * Note: data_push_tail:D and desc_reserve:G can be
> +			 *       different CPUs. However, the desc_reserve:G
> +			 *       CPU (which performs the full memory barrier)
> +			 *       must have previously seen data_push_tail:D.
> +			 */
> +			smp_rmb(); /* LMM(data_push_tail:A) */
> +
> +			next_lpos = atomic_long_read(&data_ring->tail_lpos
> +						); /* LMM(data_push_tail:B) */
> +			if (next_lpos == tail_lpos)
> +				return false;
> +
> +			/* Another task pushed the tail. Try again. */
> +			tail_lpos = next_lpos;
> +			continue;
> +		}
> +
> +		/*
> +		 * Guarantee any descriptor states that have transitioned to
> +		 * reusable are stored before pushing the tail lpos. This
> +		 * allows readers to identify if data has expired while
> +		 * reading the descriptor. This pairs with desc_read:D.
> +		 */
> +		smp_mb(); /* LMM(data_push_tail:C) */

The comment does not explain why we need a full barrier here. It talks
about writing descriptor states. It suggests that write barrier should
be enough.

I guess that this is related to the discussion that we had last time,
and the litmus test mentioned in
see https://lore.kernel.org/r/87h7zcjkxy.fsf@xxxxxxxxxxxxx

I would add something like:

		* Full barrier is necessary because the descriptors
		* might have been made reusable also by other CPUs.

For people like me, it would be great to add also link to a more
detailed explanation, for example, the litmus tests, or something
even more human readable ;-) I think that it is a "rather" common
problem. I wonder whether it is already documented somewhere.

> +	} while (!atomic_long_try_cmpxchg_relaxed(&data_ring->tail_lpos,
> +			&tail_lpos, next_lpos)); /* LMM(data_push_tail:D) */
> +
> +	return true;
> +}
> +

Best Regards,
Petr