Re: [PATCH RT] printk: console must not schedule for drivers

From: Sebastian Andrzej Siewior
Date: Wed Apr 15 2020 - 12:34:25 EST


On 2020-04-06 23:22:17 [+0200], John Ogness wrote:
> 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.

Interesting. So you get a report and I don't?

I have also patch related questions :)
- So before the re-rewrite, console_unlock() set this 0 before invoking
the console drivers so it was always 0. I assume it was called with
disabled interrupts.

- Is there a scenario in fbcon where this function is invoked and
console_may_schedule is not 0?

Sebastian