linux/drivers/tty
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
..
hvc xen: introduce xen_console_io option 2026-01-14 14:43:57 +01:00
ipwireless treewide, timers: Rename from_timer() to timer_container_of() 2025-06-08 09:07:37 +02:00
serdev serdev: Drop dev_pm_domain_detach() call 2025-09-06 15:48:38 +02:00
serial serial: Fix not set tty->port race condition 2026-01-23 17:23:09 +01:00
vt printk, vt, fbcon: Remove console_conditional_schedule() 2026-02-14 11:09:47 +01:00
amiserial.c tty: amiserial: Fix namespace collision and startup() section placement with -ffunction-sections 2025-11-21 10:04:09 +01:00
ehv_bytechan.c tty: ehv_bytechan: convert to u8 and size_t 2023-12-08 12:02:37 +01:00
goldfish.c Get rid of 'remove_new' relic from platform driver struct 2024-12-01 15:12:43 -08:00
Kconfig TTY/Serial driver updates for 6.15-rc1 2025-04-02 18:17:33 -07:00
Makefile
mips_ejtag_fdc.c treewide, timers: Rename from_timer() to timer_container_of() 2025-06-08 09:07:37 +02:00
moxa.c tty: moxa: use guard()s 2025-11-21 18:30:40 +01:00
mxser.c mxser: use guard()s 2025-08-17 12:46:26 +02:00
n_gsm.c tty: document @dlci parameter in gsm_modem_send_initial_msc 2025-10-28 15:25:00 +01:00
n_hdlc.c tty: n_hdlc: use guard()s 2025-11-21 18:30:40 +01:00
n_null.c tty: ldops: unify to u8 2023-08-11 21:12:47 +02:00
n_tty.c tty: n_tty: use guard()s 2025-11-21 18:30:40 +01:00
nozomi.c tty: nozomi: convert to u8 and size_t 2023-12-08 12:02:38 +01:00
pty.c TTY/Serial changes for 6.19-rc1 2025-12-06 18:38:19 -08:00
rpmsg_tty.c tty: make tty_operations::write()'s count size_t 2023-08-11 21:12:46 +02:00
synclink_gt.c tty: synclink_gt: Fix namespace collision and startup() section placement with -ffunction-sections 2025-11-21 10:04:10 +01:00
sysrq.c tty: sysrq: delete unnecessary check 2025-08-13 17:42:26 +02:00
tty.h tty: convert THROTTLE constants into enum 2023-10-03 14:31:16 +02:00
tty_audit.c tty: audit: do not use N_TTY_BUF_SIZE 2025-03-20 08:00:50 -07:00
tty_baudrate.c
tty_buffer.c tty: replace use of system_unbound_wq with system_dfl_wq 2025-11-26 13:11:09 +01:00
tty_io.c tty: use lock guard()s in tty_io 2025-04-25 13:46:31 +02:00
tty_ioctl.c tty: simplify throttling using guard()s 2025-04-25 13:46:30 +02:00
tty_jobctrl.c tty: tty_jobctrl: fix pid memleak in disassociate_ctty() 2023-09-18 11:14:43 +02:00
tty_ldisc.c tty: add the option to have a tty reject a new ldisc 2024-05-04 18:45:11 +02:00
tty_ldsem.c tty/ldsem: Remove unused ldsem_down_write_trylock 2025-02-04 14:38:42 +01:00
tty_mutex.c
tty_port.c tty: tty_port: use guard()s 2025-08-17 12:46:26 +02:00
ttynull.c tty: add missing MODULE_DESCRIPTION() macros 2024-06-24 16:10:11 +02:00
vcc.c treewide, timers: Rename from_timer() to timer_container_of() 2025-06-08 09:07:37 +02:00