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.ogn...@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
