[PATCH printk v2 0/9] fix console flushing

[John Ogness]

Hi,

While testing various flushing scenarios, I stumbled on a
couple issues that cause console flushing to fail. While
discussing the v1 [0] series, a couple more issues arose.
This series addresses all the issues:

1. The prb_next_seq() optimization caused inconsistent return
   values. Fix prb_next_seq() to the originally intended
   behavior but keep an optimization.

2. pr_flush() might not wait until the most recently stored
   printk() message if non-finalized records precede it. Fix
   pr_flush() to wait for all records to print that are at
   least reserved at the time of the call.

3. In panic, the panic messages will not print if non-finalized
   records precede them. Add a special condition so that
   readers on the panic CPU can drop non-finalized records.

4. It is possible (and easy to reproduce) a scenario where the
   console on the panic CPU hands over to a waiter of a stopped
   CPU. Do not use the handover feature in panic.

5. If messages are being dropped during panic, non-panic CPUs
   are silenced. But by then it is already too late and most
   likely the panic messages have been dropped. Change the
   non-panic CPU silencing logic to restrict non-panic CPUs
   from flooding the ringbuffer.

This series also performing some minor cleanups to remove open
coded checks about the panic context and improve documentation
language regarding data-less records.

Because of multiple refactoring done in recent history, it
would be helpful to provide the LTS maintainers with the proper
backported patches. I am happy to do this.

The changes since v1:

- Rename NO_LPOS to EMPTY_LINE_LPOS.

- Add and cleanup documentation to clarify language regarding
  data-less records and special lpos values.

- Implement a new prb_next_seq() optimization to preserve the
  intended behavior. This is essentially my rfc [1] with
  memory barriers added and based on an alternate implemenation
  suggested by pmladek [2].

- Introduce new prb_next_reserve_seq() function to return the
  sequence number after @head_id.

- Use prb_next_reserve_seq() instead of prb_next_seq() for
  pr_flush().

- Implement dropping non-finalized records in panic within
  _prb_read_valid() instead of printk_get_next_message(). This
  also makes use of the new prb_next_reserve_seq().

- Use the alternate implementation from pmladek [3] to avoid
  the handover feature in panic.

- Implement a new strategy to avoid dropping panic messages
  when non-panic CPUs are flooding the ringbuffer.

John Ogness

[0] https://lore.kernel.org/lkml/20231013204340.1112036-1-john.ogness@xxxxxxxxxxxxx
[1] https://lore.kernel.org/lkml/20231019132545.1190490-1-john.ogness@xxxxxxxxxxxxx
[2] https://lore.kernel.org/lkml/ZTkxOJbDLPy12n41@alley
[3] https://lore.kernel.org/lkml/ZS-r3QnpKzm7UVip@alley

John Ogness (8):
  printk: ringbuffer: Do not skip non-finalized records with
    prb_next_seq()
  printk: ringbuffer: Clarify special lpos values
  printk: For @suppress_panic_printk check for other CPU in panic
  printk: Add this_cpu_in_panic()
  printk: ringbuffer: Cleanup reader terminology
  printk: Wait for all reserved records with pr_flush()
  printk: Skip non-finalized records in panic
  printk: Avoid non-panic CPUs flooding ringbuffer

Petr Mladek (1):
  printk: Disable passing console lock owner completely during panic()

 kernel/printk/internal.h          |   1 +
 kernel/printk/printk.c            | 108 ++++++----
 kernel/printk/printk_ringbuffer.c | 343 +++++++++++++++++++++++++-----
 kernel/printk/printk_ringbuffer.h |  21 +-
 4 files changed, 382 insertions(+), 91 deletions(-)


base-commit: b4908d68609b57ad1ba4b80bd72c4d2260387e31
-- 
2.39.2