[PATCH] s390/fault: use wake_up_klogd() in bust_spinlocks()

Sergey Senozhatsky
2018-10-24T04:31:05Z

printk() without oops_in_progress set is potentially dangerous.
it will attempt to call into console driver, so if oops happened
while console driver port->lock spin_lock was locked on the same
CPU (NMI oops or oops from console driver), then re-entering
console driver from bust_spinlocks() will deadlock the system.

Some serial drivers have are re-entrant from oops path:

static void serial_console_write(struct console *co, const char *s,
unsigned count)
{
...
if (port->sysrq)
locked = 0;
else if (oops_in_progress)
locked = spin_trylock_irqsave(&port->lock, flags);
else
spin_lock_irqsave(&port->lock, flags);
...

uart_console_write(port, s, count, serial_console_putchar);
...
if (locked)
spin_unlock_irqrestore(&port->lock, flags);
}

So it's OK to call printk() or console_unblank() and re-enter
serial console drivers when oops_in_progress set. But once we
clear oops_in_progress serial consoles become non-reentrant.

From the comment it seems that s390 wants to just poke klogd.
There is wake_up_klogd() for this purpose, so we can replace
that printk(" ").

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
arch/s390/mm/fault.c | 11 +++--------
1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 2b8f32f56e0c..244993dc3c70 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -93,15 +93,10 @@ void bust_spinlocks(int yes)
} else {
int loglevel_save = console_loglevel;
console_unblank();
- oops_in_progress = 0;
- /*
- * OK, the message is on the console. Now we call printk()
- * without oops_in_progress set so that printk will give klogd
- * a poke. Hold onto your hats...
- */
- console_loglevel = 15;
- printk(" ");
console_loglevel = loglevel_save;
+
+ oops_in_progress = 0;
+ wake_up_klogd();
}
}

--
2.19.1