Re: Barrier before pushing desc_ring tail: was [PATCH v2 2/3] printk: add lockless buffer

[John Ogness]

On 2020-06-11, Petr Mladek <pmladek@xxxxxxxx> wrote:
> All this relies on the fact the the full barrier is called in
> data_push_tail() and data_push_tail() is called right above.
> But there are two situations where the barrier is not called.
> It is when:
>
>   1. desc.text_blk_lpos.next already is behind data_ring->tail_lpos.
>
>      This is safe.

Yes, and I have since expanded the comment above the data_push_tail()
while loop to mention that:

	/*
	 * Loop until the tail lpos is at or beyond @lpos. This condition
	 * may already be satisfied, resulting in no full memory barrier
	 * from data_push_tail:C being performed. However, since this CPU
	 * sees the new tail lpos, any descriptor states that transitioned to
	 * the reusable state must already be visible.
	 */

>   2. desc.text_blk_lpos == INVALID_LPOS.
>
>      It seems that this is not synchronized and other CPUs might see
>      the old state.

Great catch! This could trigger the WARN_ON at desc_reserve:F and lead
to prb_reserve() unnecessarily failing.

> The question is what to do with the empty data case. I see three
> possibilities:
>
>   1. Ignore the case with empty block because it (probably) does not
>      cause real problems.

It could cause dropped messages.

>   2. Call the full barrier in data_push_tail() even when the data
>      block is empty.

This is the common case, since most records will not have dictionary
data.

>   3. Call the full barrier also in desc_push_tail() as I suggested.
>
> My opinion:
>
> I prefer 3rd solution.

Agreed. For my next version I am folding all smp_mb() and smp_wmb()
calls into their neighboring cmpxchg_relaxed(). This would apply here as
well, changing desc_push_tail:B to a full cmpxchg().

We still need the full memory barrier at data_push_tail:C. Readers rely
on it to be able to verify if their copied data is still valid:

CPU0 (writer0)        CPU1 (writer1)       CPU2 (reader)
                                           desc_read->committed
desc_make_reusable
smp_mb
data_push_tail
                      read new data tail
                      data_push_head
                      smp_mb
                      write new data
                                           read garbage new data
                                           desc_read->reusable

John Ogness