Hi, This is the next part of our printk-rework effort (points 3 and 4 of the LPC 2019 summary [0]). Here the concept of "atomic consoles" is introduced through a new (optional) write_atomic() callback for console drivers. This callback must be implemented as an NMI-safe variant of the write() callback, meaning that it can function from any context without relying on questionable tactics such as ignoring locking and also without relying on the synchronization of console semaphore. As an example of how such an atomic console can look like, this series implements write_atomic() for the 8250 UART driver. This series also introduces a new console printing mode called "sync mode" that is only activated when the kernel is about to end (such as panic, oops, shutdown, reboot). Sync mode can only be activated if atomic consoles are available. A system without registered atomic consoles will be unaffected by this series. When in sync mode, the console printing behavior becomes: - only consoles implementing write_atomic() will be called - printing occurs within vprintk_store() instead of console_unlock(), since the console semaphore is irrelevant for atomic consoles For systems that have registered atomic consoles, this series improves the reliability of seeing crash messages by using new locking techniques rather than "ignoring locks and hoping for the best". In particular, atomic consoles rely on the CPU-reentrant spinlock (i.e. the printk cpulock) for synchronizing console output. John Ogness [0] https://lore.kernel.org/lkml/87k1acz5rx.fsf@xxxxxxxxxxxxx/ John Ogness (10): printk: relocate printk cpulock functions printk: rename printk cpulock API and always disable interrupts kgdb: delay roundup if holding printk cpulock printk: relocate printk_delay() printk: call boot_delay_msec() in printk_delay() printk: use seqcount_latch for console_seq console: add write_atomic interface printk: introduce kernel sync mode kdb: if available, only use atomic consoles for output mirroring serial: 8250: implement write_atomic arch/powerpc/include/asm/smp.h | 1 + arch/powerpc/kernel/kgdb.c | 10 +- arch/powerpc/kernel/smp.c | 5 + arch/x86/kernel/kgdb.c | 9 +- drivers/tty/serial/8250/8250.h | 47 ++- drivers/tty/serial/8250/8250_core.c | 17 +- drivers/tty/serial/8250/8250_fsl.c | 9 + drivers/tty/serial/8250/8250_ingenic.c | 7 + drivers/tty/serial/8250/8250_mtk.c | 29 +- drivers/tty/serial/8250/8250_port.c | 92 ++-- drivers/tty/serial/8250/Kconfig | 1 + include/linux/console.h | 32 ++ include/linux/kgdb.h | 3 + include/linux/printk.h | 57 +-- include/linux/serial_8250.h | 5 + kernel/debug/debug_core.c | 45 +- kernel/debug/kdb/kdb_io.c | 16 + kernel/printk/printk.c | 554 +++++++++++++++++-------- lib/Kconfig.debug | 3 + lib/dump_stack.c | 4 +- lib/nmi_backtrace.c | 4 +- 21 files changed, 684 insertions(+), 266 deletions(-) base-commit: 23d8adcf8022b9483605531d8985f5b77533cb3a -- 2.20.1
