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linux/kernel/printk
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Sebastian Andrzej Siewior 8e9bf8b9e8 printk, vt, fbcon: Remove console_conditional_schedule() 
do_con_write(), fbcon_redraw.*() invoke console_conditional_schedule()
which is a conditional scheduling point based on printk's internal
variables console_may_schedule. It may only be used if the console lock
is acquired for instance via console_lock() or console_trylock().

Prinkt sets the internal variable to 1 (and allows to schedule)
if the console lock has been acquired via console_lock(). The trylock
does not allow it.

The console_conditional_schedule() invocation in do_con_write() is
invoked shortly before console_unlock().
The console_conditional_schedule() invocation in fbcon_redraw.*()
original from fbcon_scroll() / vt's con_scroll() which originate from a
line feed.

In console_unlock() the variable is set to 0 (forbids to schedule) and
it tries to schedule while making progress printing. This is brand new
compared to when console_conditional_schedule() was added in v2.4.9.11.

In v2.6.38-rc3, console_unlock() (started its existence) iterated over
all consoles and flushed them with disabled interrupts. A scheduling
attempt here was not possible, it relied that a long print scheduled
before console_unlock().

Since commit 8d91f8b153 ("printk: do cond_resched() between lines
while outputting to consoles"), which appeared in v4.5-rc1,
console_unlock() attempts to schedule if it was allowed to schedule
while during console_lock(). Each record is idealy one line so after
every line feed.

This console_conditional_schedule() is also only relevant on
PREEMPT_NONE and PREEMPT_VOLUNTARY builds. In other configurations
cond_resched() becomes a nop and has no impact.

I'm bringing this all up just proof that it is not required anymore. It
becomes a problem on a PREEMPT_RT build with debug code enabled because
that might_sleep() in cond_resched() remains and triggers a warnings.
This is due to

 legacy_kthread_func-> console_flush_one_record ->  vt_console_print-> lf
   -> con_scroll -> fbcon_scroll

and vt_console_print() acquires a spinlock_t which does not allow a
voluntary schedule. There is no need to fb_scroll() to schedule since
console_flush_one_record() attempts to schedule after each line.
!PREEMPT_RT is not affected because the legacy printing thread is only
enabled on PREEMPT_RT builds.

Therefore I suggest to remove console_conditional_schedule().

Cc: Simona Vetter <simona@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Fixes: 5f53ca3ff8 ("printk: Implement legacy printer kthread for PREEMPT_RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Petr Mladek <pmladek@suse.com> # from printk() POV
Signed-off-by: Helge Deller <deller@gmx.de>
2026-02-14 11:09:47 +01:00
..
.kunitconfig printk: ringbuffer: Add KUnit test 2025-06-18 16:42:42 +02:00
braille.c printk: Replace strncmp() with str_has_prefix() 2019-08-16 09:54:08 +02:00
braille.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
console_cmdline.h printk: Add match_devname_and_update_preferred_console() 2024-07-04 15:41:44 +02:00
index.c kernel/printk/index.c: fix memory leak with using debugfs_lookup() 2023-02-03 10:42:02 +01:00
internal.h printk: Add execution context (task name/CPU) to printk_info 2026-02-10 19:51:56 -08:00
Makefile printk: ringbuffer: Add KUnit test 2025-06-18 16:42:42 +02:00
nbcon.c Networking changes for 7.0 2026-02-11 19:31:52 -08:00
printk.c printk, vt, fbcon: Remove console_conditional_schedule() 2026-02-14 11:09:47 +01:00
printk_ringbuffer.c printk_ringbuffer: Create a helper function to decide whether more space is needed 2025-11-10 13:09:43 +01:00
printk_ringbuffer.h printk: Add execution context (task name/CPU) to printk_info 2026-02-10 19:51:56 -08:00
printk_ringbuffer_kunit_test.c printk: kunit: support offstack cpumask 2025-09-10 17:18:37 +02:00
printk_safe.c printk: Defer legacy printing when holding printk_cpu_sync 2024-12-16 13:26:31 +01:00
sysctl.c treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00