Re: [PATCH printk v4 04/14] printk: ringbuffer: Do not skip non-finalized records with prb_next_seq()

[Petr Mladek]

On Wed 2024-02-07 14:46:53, John Ogness wrote:
> Commit f244b4dc53e5 ("printk: ringbuffer: Improve
> prb_next_seq() performance") introduced an optimization for
> prb_next_seq() by using best-effort to track recently finalized
> records. However, the order of finalization does not
> necessarily match the order of the records. The optimization
> changed prb_next_seq() to return inconsistent results, possibly
> yielding sequence numbers that are not available to readers
> because they are preceded by non-finalized records or they are
> not yet visible to the reader CPU.
> 
> Rather than simply best-effort tracking recently finalized
> records, force the committing writer to read records and
> increment the last "contiguous block" of finalized records. In
> order to do this, the sequence number instead of ID must be
> stored because ID's cannot be directly compared.
> 
> A new memory barrier pair is introduced to guarantee that a
> reader can always read the records up until the sequence number
> returned by prb_next_seq() (unless the records have since
> been overwritten in the ringbuffer).
> 
> This restores the original functionality of prb_next_seq()
> while also keeping the optimization.
> 
> For 32bit systems, only the lower 32 bits of the sequence
> number are stored. When reading the value, it is expanded to
> the full 64bit sequence number using the 32bit seq macros,
> which fold in the value returned by prb_first_seq().
> 
> Fixes: f244b4dc53e5 ("printk: ringbuffer: Improve prb_next_seq() performance")
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>

Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>

Best Regards,
Petr