add threaded printing + the rest

Hi,

This is v4 of a series to implement threaded console printing
as well as some other minor pieces (such as proc and sysfs
recognition of nbcon consoles). v3 is here [0].

For information about the motivation of the nbcon consoles,
please read the cover letter of the original v1 [1].

This series provides the remaining pieces of the printk
rework. All other components are either already mainline or are
currently in linux-next. In particular this series does:

- Implement dedicated printing threads per nbcon console.

- Implement forced threading of legacy consoles for PREEMPT_RT.

- Implement nbcon support for proc and sysfs console-related
  files.

- Provide a new helper function nbcon_reacquire_nobuf() to
  allow nbcon console drivers to reacquire ownership.

Note that this series does *not* provide an nbcon console
driver. That will come in a follow-up series.

Here are the changes since v3:

- In nbcon_enter_unsafe()/nbcon_exit_unsafe(), clear the write
  context buffer if ownership was lost.

- In nbcon_emit_next_record(), add a comment that lost
  ownership is detected when entering unsafe.

- For patch 02/17 ("printk: Fail pr_flush() if before
  SYSTEM_SCHEDULING"), update commit message as requested.

- For patch 04/17 ("printk: nbcon: Add context to usable() and
  emit()"), remove redundant error checking from
  nbcon_emit_next_record().

- In nbcon_alloc(), initialize @nbcon_seq to the highest
  possible value to avoid printing until the correct value is
  known.

- In nbcon_emit_next_record(), add a comment reminding that
  write_thread() is mandatory and already checked.

- In nbcon_alloc(), use WARN_ON() instead of a custom error
  message.

- Relocate nbcon_atomic_emit_one() above nbcon_kthread_func()
  so that it can be shared.

- Rename nbcon_wake_kthreads() to nbcon_kthreads_wake().

- Move device_lock()/device_unlock() usage inside
  nbcon_emit_one().

- Use nbcon_emit_one() for nbcon_legacy_emit_next_record() and
  nbcon_kthread_func().

- For patch 08/17 ("printk: nbcon: Use thread callback if in
  task context for legacy"), mention in the commit message
  about consolidating legacy and kthread printing.

- In console_is_usable(), add a comment explaining why
  @printk_kthreads_running is not checked for !use_atomic.

- In nbcon_atomic_flush_pending_con() and
  nbcon_device_release(), check for !ft.nbcon_offload to
  continue flushing.

- In console_start(), find out if it is an nbcon console while
  holding the console_list_lock. This avoids the need to take
  the console_srcu_read_unlock later in the function.

- In is_printk_deferred(), check force_legacy_kthread().

John Ogness

[0] https://lore.kernel.org/lkml/20240722171939.3349410-1-joh...

[1] https://lore.kernel.org/lkml/20230302195618.156940-1-john...

John Ogness (16):
  printk: nbcon: Add function for printers to reacquire ownership
  printk: Fail pr_flush() if before SYSTEM_SCHEDULING
  printk: Flush console on unregister_console()
  printk: nbcon: Add context to usable() and emit()
  printk: nbcon: Init @nbcon_seq to highest possible
  printk: nbcon: Relocate nbcon_atomic_emit_one()
  printk: nbcon: Use thread callback if in task context for legacy
  printk: nbcon: Rely on kthreads for normal operation
  printk: Provide helper for message prepending
  printk: nbcon: Show replay message on takeover
  proc: consoles: Add notation to c_start/c_stop
  proc: Add nbcon support for /proc/consoles
  tty: sysfs: Add nbcon support for 'active'
  printk: Implement legacy printer kthread for PREEMPT_RT
  printk: nbcon: Assign nice -20 for printing threads
  printk: Avoid false positive lockdep report for legacy printing

Thomas Gleixner (1):
  printk: nbcon: Introduce printer kthreads

 drivers/tty/tty_io.c              |   2 +-
 fs/proc/consoles.c                |   7 +-
 include/linux/console.h           |  48 +++
 kernel/printk/internal.h          |  82 ++++-
 kernel/printk/nbcon.c             | 497 +++++++++++++++++++++++++-----
 kernel/printk/printk.c            | 467 ++++++++++++++++++++++++----
 kernel/printk/printk_ringbuffer.h |   2 +
 kernel/printk/printk_safe.c       |   4 +-
 8 files changed, 976 insertions(+), 133 deletions(-)


base-commit: 59cd94ef80094857f0d0085daa2e32badc4cddf4
-- 
2.39.2