[PATCH v2 1/3] serial: core: introduce guard(uart_port_lock_check_sysrq_irqsave)

[Jacques Nilo]

uart_handle_break() and uart_prepare_sysrq_char() (in
include/linux/serial_core.h) capture a SysRq character into
port->sysrq_ch while the port lock is held and rely on the unlock
helper -- uart_unlock_and_check_sysrq_irqrestore() -- to dispatch the
captured character to handle_sysrq() on scope exit.

The existing guard(uart_port_lock_irqsave) cannot be used by IRQ
handlers that process RX, because its destructor calls plain
uart_port_unlock_irqrestore() and silently drops port->sysrq_ch.

Add a dedicated guard(uart_port_lock_check_sysrq_irqsave) variant
whose destructor is the sysrq-aware unlock helper. The lock side is
identical to uart_port_lock_irqsave -- only the unlock-time behaviour
differs. Callers that may capture SysRq characters must use
guard(uart_port_lock_check_sysrq_irqsave); the existing
guard(uart_port_lock_irqsave) keeps its current plain-unlock semantics
for the many callers that do not process RX.

The new macro is placed after the CONFIG_MAGIC_SYSRQ_SERIAL block so
both definitions of uart_unlock_and_check_sysrq_irqrestore() (sysrq
enabled and disabled) are visible at expansion time. When
CONFIG_MAGIC_SYSRQ_SERIAL=n the destructor degenerates to plain
uart_port_unlock_irqrestore(), so there is no overhead.

No functional change on its own; users are converted in the following
patches.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jacques Nilo <jnilo@xxxxxxx>
---
 include/linux/serial_core.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 4f7bbdd90..d1404c97d 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -1286,6 +1286,18 @@ static inline void uart_unlock_and_check_sysrq_irqrestore(struct uart_port *port
 }
 #endif	/* CONFIG_MAGIC_SYSRQ_SERIAL */
 
+/*
+ * Variant of guard(uart_port_lock_irqsave) for IRQ handlers that may capture
+ * a SysRq character via uart_prepare_sysrq_char(). The destructor uses the
+ * sysrq-aware unlock helper so that a captured port->sysrq_ch is dispatched
+ * to handle_sysrq() on scope exit. The plain guard variant silently drops
+ * sysrq_ch and must not be used by callers that process RX.
+ */
+DEFINE_LOCK_GUARD_1(uart_port_lock_check_sysrq_irqsave, struct uart_port,
+                    uart_port_lock_irqsave(_T->lock, &_T->flags),
+                    uart_unlock_and_check_sysrq_irqrestore(_T->lock, _T->flags),
+                    unsigned long flags);
+
 /*
  * We do the SysRQ and SAK checking like this...
  */
-- 
2.43.0