[PATCH printk v2 00/38] reduce console_lock scope

[PATCH printk v2 00/38] reduce console_lock scope

[John Ogness]

This is v2 of a series to prepare for threaded/atomic
printing. It is a rework of patches 6-12 of the v1 [0]. From
the v1, patches 1-5 are already mainline and a rework of
patches >12 will be posted in a later series.

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 and is completely removed from
(un)register_console() and console_stop/start() code.

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

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 v2:

general:

- introduce console_is_enabled() to document safe data race on
  console->flags

- switch all "console->flags & CON_ENABLED" code sites to
  console_is_enabled()

- add "for_each_console_srcu" to .clang-format

- cleanup/clarify comments relating to console_lock
  coverage/usage

um:

- kmsg_dumper: use srcu instead of console_lock for list
  iteration

kgdb/kdb:

- configure_kgdboc: keep console_lock for console->device()
  synchronization, use srcu for list iteration

- kgdboc_earlycon_pre_exp_handler: use srcu instead of
  documenting unsafety for list iteration

- kgdboc_earlycon_init: use console_list_lock instead of
  console_lock to lock list

- kdb_msg_write: use srcu instead of documenting unsafety for
  list iteration

tty:

- show_cons_active: keep console_lock for console->device()
  synchronization

fbdev:

- xen-fbfront: xenfb_probe: use srcu instead of console_lock
  for list iteration, introduce console_force_preferred() to
  safely implement hack

proc/consoles:

- show_console_dev: keep console_lock for console->device()
  synchronization

- c_next: use hlist_entry_safe() instead of
  hlist_for_each_entry_continue()

printk:

- remove console_lock from console_stop/start() and
  (un)register_console()

- introduce console_srcu_read_(un)lock() to wrap scru read
  (un)lock

- rename cons_first() macro to console_first()

- for_each_console: add lockdep check instead of introducing
  new for_each_registered_console()

- console_list_lock: add warning if in read-side critical
  section

- release srcu read lock on handover

- console_flush_all: use srcu instead of relying on console
  lock for list iteration

- console_unblank: use srcu instead of relying on console_lock
  for list iteration

- console_flush_on_panic: use srcu for list iteration and
  document console->seq race

- device: keep console_lock for console->device()
  synchronization, usr srcu for list iteration

- register_console: split list adding logic into the 3 distinct
  scenarios

- register_console: set initial sequence number before adding
  to list

- unregister_console: fix ENODEV return value if the console is
  not registered

- console_stop: synchronize srcu

- printk_late_init: use _safe variant of iteration

- __pr_flush: use srcu instead of relying on console_lock for
  list iteration

John Ogness

[0] https://lore.kernel.org/r/20220924000454.3319186-1-john.ogness@linutronix.de
[1] https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/log/?h=srcunmisafe.2022.10.18b

John Ogness (37):
  printk: Convert console_drivers list to hlist
  printk: Prepare for SRCU console list protection
  printk: introduce console_is_enabled() wrapper
  printk: use console_is_enabled()
  tty: nfcon: use console_is_enabled()
  um: kmsg_dump: use console_is_enabled()
  efi: earlycon: use console_is_enabled()
  netconsole: use console_is_enabled()
  tty: hvc: use console_is_enabled()
  tty: serial: earlycon: use console_is_enabled()
  tty: serial: kgdboc: use console_is_enabled()
  tty: serial: pic32_uart: use console_is_enabled()
  tty: serial: samsung_tty: use console_is_enabled()
  tty: serial: serial_core: use console_is_enabled()
  tty: serial: xilinx_uartps: use console_is_enabled()
  tty: tty_io: use console_is_enabled()
  usb: early: xhci-dbc: use console_is_enabled()
  kdb: kdb_io: use console_is_enabled()
  um: kmsg_dumper: use srcu console list iterator
  serial: kgdboc: use srcu console list iterator
  serial: kgdboc: document console_lock usage
  tty: tty_io: document console_lock usage
  xen: fbfront: use srcu console list iterator
  proc: consoles: document console_lock usage
  kdb: use srcu console list iterator
  printk: console_flush_all: use srcu console list iterator
  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
  printk: register_console: use srcu console list iterator
  printk: __pr_flush: use srcu console list iterator
  printk: introduce console_list_lock
  serial: kgdboc: use console_list_lock instead of console_lock
  tty: tty_io: use console_list_lock for list synchronization
  proc: consoles: use console_list_lock for list iteration
  printk: relieve console_lock of list synchronization duties
  printk, xen: fbfront: create/use safe function for forcing preferred

Thomas Gleixner (1):
  serial: kgdboc: Lock console list in probe function

 .clang-format                      |   1 +
 arch/m68k/emu/nfcon.c              |   4 +-
 arch/um/kernel/kmsg_dump.c         |  15 +-
 drivers/firmware/efi/earlycon.c    |   4 +-
 drivers/net/netconsole.c           |   4 +-
 drivers/tty/hvc/hvc_console.c      |   2 +-
 drivers/tty/serial/earlycon.c      |   4 +-
 drivers/tty/serial/kgdboc.c        |  37 ++-
 drivers/tty/serial/pic32_uart.c    |   2 +-
 drivers/tty/serial/samsung_tty.c   |   2 +-
 drivers/tty/serial/serial_core.c   |   2 +-
 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  |  16 +-
 fs/proc/consoles.c                 |  20 +-
 include/linux/console.h            |  75 +++++-
 include/linux/serial_core.h        |   2 +-
 kernel/debug/kdb/kdb_io.c          |   7 +-
 kernel/printk/printk.c             | 373 +++++++++++++++++++++--------
 20 files changed, 438 insertions(+), 154 deletions(-)


base-commit: c2d158a284abd63d727dad7402a2eed650dd4233
-- 
2.30.2

[PATCH printk v2 10/38] tty: hvc: use console_is_enabled()

[John Ogness]

Replace (console->flags & CON_ENABLED) usage with console_is_enabled().

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/hvc/hvc_console.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 4802cfaa107f..6d1d7b72488c 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -265,7 +265,7 @@ static void hvc_port_destruct(struct tty_port *port)
 static void hvc_check_console(int index)
 {
 	/* Already enabled, bail out */
-	if (hvc_console.flags & CON_ENABLED)
+	if (console_is_enabled(&hvc_console))
 		return;
 
  	/* If this index is what the user requested, then register
-- 
2.30.2

Re: [PATCH printk v2 10/38] tty: hvc: use console_is_enabled()

[Greg Kroah-Hartman]

On Wed, Oct 19, 2022 at 05:01:32PM +0206, John Ogness wrote:
> Replace (console->flags & CON_ENABLED) usage with console_is_enabled().
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Re: [PATCH printk v2 10/38] tty: hvc: use console_is_enabled()

[Petr Mladek]

On Wed 2022-10-19 17:01:32, John Ogness wrote:
> Replace (console->flags & CON_ENABLED) usage with console_is_enabled().
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
>  drivers/tty/hvc/hvc_console.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
> index 4802cfaa107f..6d1d7b72488c 100644
> --- a/drivers/tty/hvc/hvc_console.c
> +++ b/drivers/tty/hvc/hvc_console.c
> @@ -265,7 +265,7 @@ static void hvc_port_destruct(struct tty_port *port)
>  static void hvc_check_console(int index)
>  {
>  	/* Already enabled, bail out */
> -	if (hvc_console.flags & CON_ENABLED)
> +	if (console_is_enabled(&hvc_console))
>  		return;

The check is not reliable. The console might be disabled even when
it is already registered.

I would be nice to fix this. But it might be done later.
Feel free to use:

Reviewed-by: Petr Mladek <pmladek@suse.com>

>  
>   	/* If this index is what the user requested, then register

Best Regards,
Petr