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@xxxxxxxxxxxxx [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
