patches/printk-console-must-not-schedule-for-drivers.patch

From: John Ogness <john.ogness@linutronix.de>
Date: Mon, 6 Apr 2020 23:22:17 +0200
Subject: [PATCH] printk: console must not schedule for drivers

Even though the printk kthread is always preemptible, it is still not
allowed to call cond_resched() from within console drivers. The
task may become non-preemptible in the console driver call chain. For
example, vt_console_print() takes a spinlock and then can call into
fbcon_redraw(), which can conditionally invoke cond_resched():

|BUG: sleeping function called from invalid context at kernel/printk/printk.c:2322
|in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 177, name: printk
|CPU: 0 PID: 177 Comm: printk Not tainted 5.6.2-00011-ga536059557f1d9 #1
|Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
|Call Trace:
| dump_stack+0x66/0x8b
| ___might_sleep+0x102/0x120
| console_conditional_schedule+0x24/0x30
| fbcon_redraw+0x96/0x1c0
| fbcon_scroll+0x556/0xd70
| con_scroll+0x147/0x1e0
| lf+0x9e/0xb0
| vt_console_print+0x253/0x3d0
| printk_kthread_func+0x1d5/0x3b0

Disable cond_resched() for the call into the console drivers.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/printk/printk.c |    1 +
 1 file changed, 1 insertion(+)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3059,6 +3059,7 @@ static int printk_kthread_func(void *dat
 			    &len, printk_time);
 
 		console_lock();
+		console_may_schedule = 0;
 		if (len > 0 || ext_len > 0) {
 			call_console_drivers(ext_text, ext_len, text, len);
 			boot_delay_msec(msg->level);