# \[PATCH printk v4 00/39\] reduce console\_lock scope - Subsystems: HYPERVISOR VIRTUAL CONSOLE DRIVER, 8250/16?50 (AND CLONE UARTS) SERIAL DRIVER, ARM/SAMSUNG S3C, S5P AND EXYNOS ARM ARCHITECTURES, CLANG-FORMAT FILE, CONSOLE SUBSYSTEM, EXTENSIBLE FIRMWARE INTERFACE (EFI), FILESYSTEMS (VFS and infrastructure), FRAMEBUFFER LAYER, KGDB / KDB /debug\_core, M68K ARCHITECTURE, NETCONSOLE, NETWORKING DRIVERS, PRINTK, PROC FILESYSTEM, THE REST, TTY LAYER AND SERIAL DRIVERS, USB SUBSYSTEM, USER-MODE LINUX (UML), XEN HYPERVISOR INTERFACE - Maintainers: Greg Kroah-Hartman, Krzysztof Kozlowski, Peter Griffin, Miguel Ojeda, Ard Biesheuvel, Alexander Viro, Christian Brauner, Helge Deller, Jason Wessel, Daniel Thompson, Geert Uytterhoeven, Breno Leitao, Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Petr Mladek, John Ogness, Linus Torvalds, Jiri Slaby, Richard Weinberger, Anton Ivanov, Johannes Berg, Juergen Gross, Stefano Stabellini - Series: 39 patches touching 22 files, +703/-237 - Status: SUPERSEDED (Superseded by v5 posted 2022-11-16) - Superseded by v5 (posted 2022-11-16). - Revision v4 (4 revisions archived). - List: linuxppc-dev - 2 messages, 1 author - Page: https://ratatoskr.run/linuxppc-dev/2022/11/238613/t ## \[PATCH printk v4 00/39\] reduce console\_lock scope - From: John Ogness \ - Date: 2022-11-14 16:29:41 - Message: https://ratatoskr.run/linuxppc-dev/2022/11/238613.md ```text 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.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: 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 ``` ## \[PATCH printk v4 24/39\] tty: hvc: use console\_is\_registered() - From: John Ogness \ - Date: 2022-11-14 16:31:13 - Message: https://ratatoskr.run/linuxppc-dev/2022/11/2005003.md ```text It is not reliable to check for CON_ENABLED in order to identify if a console is registered. Use console_is_registered() instead. Signed-off-by: John Ogness Reviewed-by: Petr Mladek --- drivers/tty/hvc/hvc_console.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c index 4802cfaa107f..a683e21df19c 100644 --- a/drivers/tty/hvc/hvc_console.c +++ b/drivers/tty/hvc/hvc_console.c @@ -264,8 +264,8 @@ 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) + /* Already registered, bail out */ + if (console_is_registered(&hvc_console)) return; /* If this index is what the user requested, then register -- 2.30.2 ```