[PATCH 0/2] sched: Introduce and use deferred WARNs in sched

[Sebastian Andrzej Siewior]

This is a follow-up to the netconsole lockup reported
	https://lore.kernel.org/all/20260610183621.3915271-1-vlad.wing@xxxxxxxxx/

The idea is to use deferred printing for WARNs and use them in sched. I
tried to use only where it looks that the rq lock acquired instead a
plain s/WARN_ON/WARN_ON_DEFFERED which would be simpler.

This unholy deferred mess can be removed once we don't have legacy
consoles anymore _or_ force force_legacy_kthread=true.

The initial report is against v6.16 and netconsole. The reported problem
does not occur upstream since commit 7eab73b18630e ("netconsole: convert
to NBCON console infrastructure") which is v7.0-rc1.

Should this be rejected outright because the preferred sollution is to
| - stick msg in buffer (lockless)
| - print to atomic consoles (lockless)
| - use irq_work to wake console kthreads (lockless)
| - each kthread then tries to flush buffer to its own non-atomic console
|   in non-atomic context."

then this means to force force_legacy_kthread=true.
The threaded legacy printer is available since v6.12-rc1. It terms of stable
fix, this could go back as of v6.12 stable and not earlier (in case we care).

I tested this on a x86 box with 8250 and warning in put_prev_entity().
After it printed the initial warning, it dead-locked shortly after
because systemd was writing to the kernel buffer it acquired the
uart_port_lock then attempted to write lockdep report which required the
same lock…

Sebastian Andrzej Siewior (2):
  bug: Provide WARN_ON.*DEFERRED() macros for console deferred output
  sched: Use WARN_ON.*_DEFERRED()

 include/asm-generic/bug.h  |  41 ++++++++++++++
 kernel/sched/core.c        |  78 +++++++++++++-------------
 kernel/sched/core_sched.c  |   6 +-
 kernel/sched/cpudeadline.c |   6 +-
 kernel/sched/deadline.c    |  62 ++++++++++-----------
 kernel/sched/ext.c         | 110 ++++++++++++++++++-------------------
 kernel/sched/fair.c        |  88 ++++++++++++++---------------
 kernel/sched/rt.c          |  36 ++++++------
 kernel/sched/sched.h       |  18 +++---
 lib/bug.c                  |  16 +++++-
 10 files changed, 257 insertions(+), 204 deletions(-)

-- 
2.53.0