[PATCH printk v4 00/39] reduce console_lock scope

[John Ogness]

This is v4 of a series to prepare for threaded/atomic
printing. v3 is here [0]. This series focuses on reducing the
scope of the BKL console_lock. It achieves this by switching to
SRCU and a dedicated mutex for console list iteration and
modification, respectively. The console_lock will no longer
offer this protection.

Also, during the review of v2 it came to our attention that
many console drivers are checking CON_ENABLED to see if they
are registered. Because this flag can change without
unregistering and because this flag does not represent an
atomic point when an (un)registration process is complete,
a new console_is_registered() function is introduced. This
function uses the console_list_lock to synchronize with the
(un)registration process to provide a reliable status.

All users of the console_lock for list iteration have been
modified. For the call sites where the console_lock is still
needed (for other reasons), comments are added to explain
exactly why the console_lock was needed.

All users of CON_ENABLED for registration status have been
modified to use console_is_registered(). Note that there are
still users of CON_ENABLED, but this is for legitimate purposes
about a registered console being able to print.

The base commit for this series is from Paul McKenney's RCU tree
and provides an NMI-safe SRCU implementation [1]. Without the
NMI-safe SRCU implementation, this series is not less safe than
mainline. But we will need the NMI-safe SRCU implementation for
atomic consoles anyway, so we might as well get it in
now. Especially since it _does_ increase the reliability for
mainline in the panic path.

Changes since v3:

general:

- Implement console_srcu_read_flags() and console_srcu_write_flags()
  to be used for console->flags access under the srcu_read_lock or
  console_list_lock, respectively. The functions document their
  relationship to one another and use data_race(), READ_ONCE(), and
  WRITE_ONCE() macros to annotate their relationship. They also make
  use of lockdep to warn if used in improper contexts.

- Replace all console_is_enabled() usage with
  console_srcu_read_flags() (all were under the srcu_read_lock).

serial_core:

- For uart_console_registered(), check uart_console() before taking
  the console_list_lock to avoid unnecessary lock contention for
  non-console ports.

m68k/emu/nfcon:

- Only explicitly enable the console if registering via debug=nfcon.

tty/serial/sh-sci:

- Add comments about why @options will always be NULL for the
  earlyprintk console.

kdb:

- Add comments explaining the expectations for console drivers to
  work correctly.

printk:

- Some code sites under SRCU were checking flags directly. Use
  console_srcu_read_flags() instead.

- In register_console() rename bootcon_enabled/realcon_enabled to
  bootcon_registered/realcon_registered to avoid confusion.

- In register_console() only check for boot console sequences if a
  boot console is registered and !keep_bootcon. In this case, also
  take the console_lock to guarantee safe access to console->seq.

- In console_force_preferred_locked() use hlist_del_rcu() instead of
  hlist_del_init_rcu() so that there is never a window where the
  console can be viewed as unregistered.

John Ogness

[0] https://lore.kernel.org/lkml/20221107141638.3790965-1-john.ogness@linutronix.de
[1] https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/log/?h=srcunmisafe.2022.11.09a

John Ogness (37):
  printk: Prepare for SRCU console list protection
  printk: register_console: use "registered" for variable names
  printk: fix setting first seq for consoles
  um: kmsg_dump: only dump when no output console available
  tty: serial: kgdboc: document console_lock usage
  tty: tty_io: document console_lock usage
  proc: consoles: document console_lock usage
  printk: introduce console_list_lock
  console: introduce wrappers to read/write console flags
  um: kmsg_dumper: use srcu console list iterator
  kdb: use srcu console list iterator
  printk: console_flush_all: use srcu console list iterator
  printk: __pr_flush: use srcu console list iterator
  printk: console_is_usable: use console_srcu_read_flags
  printk: console_unblank: use srcu console list iterator
  printk: console_flush_on_panic: use srcu console list iterator
  printk: console_device: use srcu console list iterator
  console: introduce console_is_registered()
  serial_core: replace uart_console_enabled() with
    uart_console_registered()
  tty: nfcon: use console_is_registered()
  efi: earlycon: use console_is_registered()
  tty: hvc: use console_is_registered()
  tty: serial: earlycon: use console_is_registered()
  tty: serial: pic32_uart: use console_is_registered()
  tty: serial: samsung_tty: use console_is_registered()
  tty: serial: xilinx_uartps: use console_is_registered()
  usb: early: xhci-dbc: use console_is_registered()
  netconsole: avoid CON_ENABLED misuse to track registration
  printk, xen: fbfront: create/use safe function for forcing preferred
  tty: tty_io: use console_list_lock for list synchronization
  proc: consoles: use console_list_lock for list iteration
  tty: serial: kgdboc: use srcu console list iterator
  tty: serial: kgdboc: use console_list_lock for list traversal
  tty: serial: kgdboc: synchronize tty_find_polling_driver() and
    register_console()
  tty: serial: kgdboc: use console_list_lock to trap exit
  printk: relieve console_lock of list synchronization duties
  tty: serial: sh-sci: use setup() callback for early console

Thomas Gleixner (2):
  serial: kgdboc: Lock console list in probe function
  printk: Convert console_drivers list to hlist

 .clang-format                       |   1 +
 arch/m68k/emu/nfcon.c               |   9 +-
 arch/um/kernel/kmsg_dump.c          |  24 +-
 drivers/firmware/efi/earlycon.c     |   8 +-
 drivers/net/netconsole.c            |  21 +-
 drivers/tty/hvc/hvc_console.c       |   4 +-
 drivers/tty/serial/8250/8250_core.c |   2 +-
 drivers/tty/serial/earlycon.c       |   4 +-
 drivers/tty/serial/kgdboc.c         |  46 ++-
 drivers/tty/serial/pic32_uart.c     |   4 +-
 drivers/tty/serial/samsung_tty.c    |   2 +-
 drivers/tty/serial/serial_core.c    |  14 +-
 drivers/tty/serial/sh-sci.c         |  20 +-
 drivers/tty/serial/xilinx_uartps.c  |   2 +-
 drivers/tty/tty_io.c                |  18 +-
 drivers/usb/early/xhci-dbc.c        |   2 +-
 drivers/video/fbdev/xen-fbfront.c   |  12 +-
 fs/proc/consoles.c                  |  21 +-
 include/linux/console.h             | 129 +++++++-
 include/linux/serial_core.h         |  10 +-
 kernel/debug/kdb/kdb_io.c           |  18 +-
 kernel/printk/printk.c              | 459 +++++++++++++++++++++-------
 22 files changed, 648 insertions(+), 182 deletions(-)


base-commit: f733615e39aa2d6ddeef33b7b2c9aa6a5a2c2785
-- 
2.30.2