[PATCH RT 1/8] printk: console must not schedule for drivers

[Steven Rostedt]

5.4.44-rt27-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: John Ogness <john.ogness@xxxxxxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 kernel/printk/printk.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2affe95866a4..9600373bcf55 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2746,6 +2746,7 @@ static int printk_kthread_func(void *data)
 			    &len, printk_time);
 
 		console_lock();
+		console_may_schedule = 0;
 		call_console_drivers(master_seq, ext_text, ext_len, text, len,
 				     msg->level, msg->facility);
 		if (len > 0 || ext_len > 0)
-- 
2.26.2