--- v2
+++ v4
@@ -1,19 +1,28 @@
-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 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.
-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), I added comments to explain
+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
@@ -23,176 +32,132 @@
now. Especially since it _does_ increase the reliability for
mainline in the panic path.
-Changes since v2:
+Changes since v3:
general:
-- introduce console_is_enabled() to document safe data race on
- console->flags
+- 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.
-- switch all "console->flags & CON_ENABLED" code sites to
- console_is_enabled()
+- Replace all console_is_enabled() usage with
+ console_srcu_read_flags() (all were under the srcu_read_lock).
-- add "for_each_console_srcu" to .clang-format
+serial_core:
-- cleanup/clarify comments relating to console_lock
- coverage/usage
+- For uart_console_registered(), check uart_console() before taking
+ the console_list_lock to avoid unnecessary lock contention for
+ non-console ports.
-um:
+m68k/emu/nfcon:
-- kmsg_dumper: use srcu instead of console_lock for list
- iteration
+- Only explicitly enable the console if registering via debug=nfcon.
-kgdb/kdb:
+tty/serial/sh-sci:
-- configure_kgdboc: keep console_lock for console->device()
- synchronization, use srcu for list iteration
+- Add comments about why @options will always be NULL for the
+ earlyprintk console.
-- kgdboc_earlycon_pre_exp_handler: use srcu instead of
- documenting unsafety for list iteration
+kdb:
-- 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()
+- Add comments explaining the expectations for console drivers to
+ work correctly.
printk:
-- remove console_lock from console_stop/start() and
- (un)register_console()
+- Some code sites under SRCU were checking flags directly. Use
+ console_srcu_read_flags() instead.
-- introduce console_srcu_read_(un)lock() to wrap scru read
- (un)lock
+- In register_console() rename bootcon_enabled/realcon_enabled to
+ bootcon_registered/realcon_registered to avoid confusion.
-- rename cons_first() macro to console_first()
+- 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.
-- 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
+- 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/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
+[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: 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()
+ 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
- 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: __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
- 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
+ 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
- printk, xen: fbfront: create/use safe function for forcing preferred
+ tty: serial: sh-sci: use setup() callback for early console
-Thomas Gleixner (1):
+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 | 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(-)
+ .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: c2d158a284abd63d727dad7402a2eed650dd4233
+base-commit: f733615e39aa2d6ddeef33b7b2c9aa6a5a2c2785
--
2.30.2