[PATCHv3] panic: avoid deadlocks in re-entrant console drivers

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

>From printk()/serial console point of view panic() is special, because
it may force CPU to re-enter printk() or/and serial console driver.
Therefore, some of serial consoles drivers are re-entrant. E.g. 8250:

serial8250_console_write()
{
        if (port->sysrq)
                locked = 0;
        else if (oops_in_progress)
                locked = spin_trylock_irqsave(&port->lock, flags);
        else
                spin_lock_irqsave(&port->lock, flags);
	...
}

panic() does set oops_in_progress via bust_spinlocks(1), so in theory
we should be able to re-enter serial console driver from panic():

	CPU0
	<NMI>
	uart_console_write()
	serial8250_console_write()		// if (oops_in_progress)
						//    spin_trylock_irqsave()
	call_console_drivers()
	console_unlock()
	console_flush_on_panic()
	bust_spinlocks(1)			// oops_in_progress++
	panic()
	<NMI/>
	spin_lock_irqsave(&port->lock, flags)   // spin_lock_irqsave()
	serial8250_console_write()
	call_console_drivers()
	console_unlock()
	printk()
	...

However, this does not happen and we deadlock in serial console on
port->lock spinlock. And the problem is that console_flush_on_panic()
called after bust_spinlocks(0):

void panic(const char *fmt, ...)
{
        bust_spinlocks(1);
...
        bust_spinlocks(0);
        console_flush_on_panic();
...
}

bust_spinlocks(0) decrements oops_in_progress, so oops_in_progress
can go back to zero. Thus even re-entrant console drivers will simply
spin on port->lock spinlock. Given that port->lock may already be
locked either by a stopped CPU, or by the very same CPU we execute
panic() on (for instance, NMI panic() on printing CPU) the system
deadlocks and does not reboot.

Fix this by removing bust_spinlocks(0), so oops_in_progress is always
set in panic() now and, thus, re-entrant console drivers will trylock
the port->lock instead of spinning on it forever, when we call them
from console_flush_on_panic().

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---

v2: do not do bust_spinlocks(1);bust_spinlocks(0);bust_spinlocks(1)
    thing and just copy paste from lib/bust_spinlocks what
    bust_spinlocks(0) does. (Petr)

 kernel/panic.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index f6d549a29a5c..fccd41628b24 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -237,7 +237,10 @@ void panic(const char *fmt, ...)
 	if (_crash_kexec_post_notifiers)
 		__crash_kexec(NULL);
 
-	bust_spinlocks(0);
+#ifdef CONFIG_VT
+	unblank_screen();
+#endif
+	console_unblank();
 
 	/*
 	 * We may have ended up stopping the CPU holding the lock (in
-- 
2.19.1