linux-kernel - [PATCH RT] printk: console must not schedule for drivers

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Message-Id: <20200406212217.2323-1-john.ogness@linutronix.de>
Date:   Mon,  6 Apr 2020 23:22:17 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: [PATCH RT] 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_cursor+0x100/0x190
 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@...el.com>
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/printk/printk.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8821a8c2263f..8bc683be0857 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2715,6 +2715,7 @@ 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)
-- 
2.19.0