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    Subject[PATCH printk v3 00/40] reduce console_lock scope
    Date
    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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