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Subject[RFC][PATCHv2 3/4] serial: introduce uart_port locking helpers
Date
The patch introduces several macros which should make serial console
drivers less printk() deadlock prone.

There are several console driver entry points so we are looking at
several slightly different deadlock scenarios.

- The first entry point is console ->write() callback, which we call
from printk(). A possible deadlock scenario there is:

CPU0
<NMI>
spin_lock_irqsave(&port->lock, flags) << deadlock
serial_foo_write()
call_console_drivers()
console_unlock()
console_flush_on_panic()
panic()
<NMI/>
spin_lock_irqsave(&port->lock, flags)
serial_foo_write()
call_console_drivers()
console_unlock()
printk()
...

->write() callback, generally speaking, must be re-entrant in two cases:
1) sysrq
2) panic (oops_in_progress)

And this is what uart_port_lock_for_printk()/uart_port_unlock_after_printk()
macros are for.

Usage example:

static serial_foo_write(...)
{
struct uart_port *port;
unsigned long flags;
bool locked;

uart_port_lock_for_printk(port, flags, locked);
uart_console_write(port, s, count, serial_foo_putchar);
uart_port_unlock_after_printk(port, flags, locked);
}

Some of the serial drivers already use _some sort_ of
uart_port_lock_for_printk(), some are not panic() re-entrant. This
should address the issue.

- The rest (of entry points) requires a bit different handling.
Let's take a look at the following backtrace:

CPU0
<IRQ>
spin_lock_irqsave(&port->lock, flags) << deadlock
serial_foo_write()
call_console_drivers()
console_unlock()
printk()
__queue_work()
tty_flip_buffer_push()
spin_lock_irqsave(&port->lock, flags)
serial_foo_handle_IRQ()
<IRQ/>

Serial drivers invoke tons of core kernel functions - WQ, MM, etc. All
of which may printk() in various cases. So we can't really just
"remove those printk-s". The simples way to address this seems to be
PRINTK_SAFE_CONTEXT_MASK.

printk_safe() is a special printk() mode, which redirects recursive
printk()-s to a secondary (per-CPU) buffer; so we don't re-enter
printk() and serial console driver. The secondary (per-CPU) buffer is
flushed and printed to the consoles later, from a safe context, when
we know that we are not in printk()-recursion anymore.

And this is what uart_port_lock_*()/uart_port_unlock_*() macros are
for. With uart_port_lock_irqsave() the previous example would turn
into:

CPU0
<IRQ>
irq_work_queue()
printk_safe_log_store()
printk()
__queue_work()
tty_flip_buffer_push()
uart_port_lock_irqsave(port, flags)
serial_foo_handle_IRQ()
<IRQ/>

As of now, no consoles are re-entrant when printk() initiated by
console's driver IRQ handler. This macro should address the issue.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
include/linux/serial_core.h | 48 +++++++++++++++++++++++++++++++++++++
1 file changed, 48 insertions(+)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 047fa67d039b..acc6966fea8e 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -266,6 +266,54 @@ struct uart_port {
void *private_data; /* generic platform data pointer */
};

+#define uart_port_lock_irq(p) \
+ do { \
+ printk_safe_enter_irq(); \
+ spin_lock(&(p)->lock); \
+ } while (0)
+
+#define uart_port_unlock_irq(p) \
+ do { \
+ spin_unlock(&(p)->lock); \
+ printk_safe_exit_irq(); \
+ } while (0)
+
+#define uart_port_lock_irqsave(p, flags) \
+ do { \
+ printk_safe_enter_irqsave(flags); \
+ spin_lock(&(p)->lock); \
+ } while (0)
+
+#define uart_port_unlock_irqrestore(p, flags) \
+ do { \
+ spin_unlock(&(p)->lock); \
+ printk_safe_exit_irqrestore(flags); \
+ } while (0)
+
+#if defined(CONFIG_SERIAL_CORE_CONSOLE) || defined(SUPPORT_SYSRQ)
+#define uart_port_in_sysrq(p) p->sysrq
+#else
+#define uart_port_in_sysrq(p) 0
+#endif
+
+#define uart_port_lock_for_printk(p, flags, locked) \
+ do { \
+ locked = true; \
+ if (uart_port_in_sysrq(p)) \
+ locked = false; \
+ else if (oops_in_progress) \
+ locked = spin_trylock_irqsave(&(p)->lock, \
+ flags); \
+ else \
+ spin_lock_irqsave(&(p)->lock, flags); \
+ } while (0)
+
+#define uart_port_unlock_after_printk(p, flags, locked) \
+ do { \
+ if (locked) \
+ spin_unlock_irqrestore(&(p)->lock, flags); \
+ } while (0)
+
static inline int serial_port_in(struct uart_port *up, int offset)
{
return up->serial_in(up, offset);
--
2.19.1
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