The legacy printer kthread uses console_lock and
__console_flush_and_unlock to flush records to the console which
holds the console_lock being held for the entire flush. This
results in large waiting times for console_lock waiters
especially where there is a large volume of records or where the
console is slow (e.g. serial). During boot, this contention causes
delays in the filp_open call in console_on_rootfs.
   
Let's instead release and reacquire console_lock in between
printing individual records.

Signed-off-by: Andrew Murray <amurray@thegoodpenguin.co.uk>
---
Changes in v2:
- Move any_usable=false to console_flush_all in the 'introduce
  console_flush_one_record' patch to match original implementation.
- Add Petr's console_flush_one_record() code cleanup patch
- Open code flushing implementation in legacy_kthread_func instead
  of introducing new console_flush functions.
- Link to v1: https://lore.kernel.org/r/20250915-printk_legacy_thread_console_lock-v1-0-f34d42a9bcb3@thegoodpenguin.co.uk

---
Andrew Murray (2):
      printk: Introduce console_flush_one_record
      printk: Use console_flush_one_record for legacy printer kthread

Petr Mladek (1):
      printk: console_flush_one_record() code cleanup

 kernel/printk/printk.c | 186 +++++++++++++++++++++++++++++++------------------
 1 file changed, 119 insertions(+), 67 deletions(-)
---
base-commit: f83ec76bf285bea5727f478a68b894f5543ca76e
change-id: 20250914-printk_legacy_thread_console_lock-1c27f59bf990

Best regards,
-- 
Andrew Murray <amurray@thegoodpenguin.co.uk>