[PATCH printk v3 00/40] reduce console_lock scope
John Ogness
john.ogness at linutronix.de
Mon Nov 7 14:15:58 UTC 2022
This is v3 of a series to prepare for threaded/atomic
printing. v2 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 and is completely removed from
(un)register_console() and console_stop/start() code.
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 (because of 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:
- introduce a synchronized console_is_registered() to query if
a console is registered, meant to replace CON_ENABLED
(mis)use for this purpose
- directly read console->flags for registered consoles if it is
race-free (and document that it is so)
- replace uart_console_enabled() with a new
uart_console_registered() based on console_is_registered()
- change comments about why console_lock is used to synchronize
console->device() by providing an example
registration check fixups:
- the following drivers were modified to use the new
console_is_registered() instead of CON_ENABLED checks
- arch/m68k/emu/nfcon.c
- drivers/firmware/efi/earlycon.c
- drivers/net/netconsole.c
- drivers/tty/hvc/hvc_console.c
- drivers/tty/serial/8250/8250_core.c
- drivers/tty/serial/earlycon.c
- drivers/tty/serial/pic32_uart.c
- drivers/tty/serial/samsung_tty.c
- drivers/tty/serial/serial_core.c
- drivers/tty/serial/xilinx_uartps.c
- drivers/usb/early/xhci-dbc.c
um: kmsg_dumper:
- change stdout dump criteria to match original intention
kgdb/kdb:
- in configure_kgdboc(), take console_list_lock to synchronize
tty_find_polling_driver() against register_console()
- add comments explaining why calling console->write() without
locking might work
tty: sh-sci:
- use a setup() callback to setup the early console
fbdev: xen:
- implement a cleaner approach for
console_force_preferred_locked()
rcu:
- implement debug_lockdep_rcu_enabled() for
!CONFIG_DEBUG_LOCK_ALLOC
printk:
- check CONFIG_DEBUG_LOCK_ALLOC for srcu_read_lock_held()
availability
- for console_lock/_trylock/_unlock, replace "lock the console
system" language with "block the console subsystem from
printing"
- use WRITE_ONCE() for updating console->flags of registered
consoles
- expand comments of synchronize_srcu() calls to explain why
they are needed, and also expand comments to explain when it
is not needed
- change CON_BOOT consoles to always begin at earliest message
- for non-BOOT/non-PRINTBUFFER consoles, initialize @seq to the
minimal @seq of any of the enabled boot consoles
- add comments and lockdep assertion to
unregister_console_locked() because it is not clear from the
name which lock is implied
- dropped patches that caused unnecessary churn in the series
John Ogness
[0] https://lore.kernel.org/lkml/20221019145600.1282823-1-john.ogness@linutronix.de
[1] https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/log/?h=srcunmisafe.2022.10.21a
John Ogness (38):
rcu: implement lockdep_rcu_enabled for !CONFIG_DEBUG_LOCK_ALLOC
printk: Prepare for SRCU console list protection
printk: fix setting first seq for consoles
um: kmsg_dump: only dump when no output console available
console: introduce console_is_enabled() wrapper
printk: use console_is_enabled()
um: kmsg_dump: use console_is_enabled()
kdb: kdb_io: use console_is_enabled()
um: kmsg_dumper: use srcu console list iterator
tty: serial: kgdboc: document console_lock usage
tty: tty_io: document console_lock usage
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: __pr_flush: use srcu console list iterator
printk: introduce console_list_lock
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 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 | 10 +-
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 | 17 +-
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 | 111 +++++++-
include/linux/rcupdate.h | 5 +
include/linux/serial_core.h | 15 +-
kernel/debug/kdb/kdb_io.c | 14 +-
kernel/printk/printk.c | 424 +++++++++++++++++++++-------
23 files changed, 605 insertions(+), 176 deletions(-)
base-commit: e29a4915db1480f96e0bc2e928699d086a71f43c
--
2.30.2
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