[PATCH 0/3] serial: 8250: fix BREAK+SysRq dispatch on guard()-locked IRQ handlers

[Jacques Nilo <jnilo@free.fr>]

This series fixes a silent regression where a SysRq character entered as
BREAK + key on the serial console is consumed by the kernel but never
dispatched to handle_sysrq().

The root cause and two fix candidates were discussed in [1]. Following
Ilpo's suggestion, this series adds a dedicated lock-guard variant whose
destructor is the sysrq-aware unlock helper, and switches the two
affected IRQ handlers (serial8250_handle_irq and dw8250_handle_irq) to
use it. The plain guard(uart_port_lock_irqsave) keeps its current
semantics for the many callers that do not process RX.

Patch 1 introduces guard(uart_port_lock_sysrq_irqsave) in serial_core.h.
Patch 2 switches serial8250_handle_irq() and updates the Context comment
        on serial8250_handle_irq_locked() so future HW-specific 8250
        wrappers know which unlock variant is required.
Patch 3 switches dw8250_handle_irq() to the same guard.

Verified on RTL8196E with CONFIG_MAGIC_SYSRQ_SERIAL=y: BREAK + 'h' on
the console UART now produces the SysRq help dump in dmesg; the brk
counter in /proc/tty/driver/serial increments per BREAK as expected.
Build tested on tty-next (base 16e95bfb79b5).

[1] https://lore.kernel.org/linux-serial/5efe9e03-4d86-43a0-9ec2-e610ff31095d@free.fr/

Jacques Nilo (3):
  serial: core: introduce guard(uart_port_lock_sysrq_irqsave)
  serial: 8250: dispatch SysRq character in serial8250_handle_irq()
  serial: 8250_dw: dispatch SysRq character in dw8250_handle_irq()

 drivers/tty/serial/8250/8250_dw.c   |  2 +-
 drivers/tty/serial/8250/8250_port.c |  7 +++++--
 include/linux/serial_core.h         | 13 +++++++++++++
 3 files changed, 19 insertions(+), 3 deletions(-)


base-commit: 16e95bfb79b5d9d01dc7651d98caf3c2ace331cd
-- 
2.43.0