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

[John Ogness <john.ogness@linutronix.de>]

On 2020-04-15, Sebastian Andrzej Siewior <bigeasy@linutronix.de> 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?

Apparently only the patch author was notified. The reporter should
include all the people tagged in the patch. I'll send him an email about
this.

> - 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.

Correct.

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

Yes. The ttys/consoles are invoked through other call chains not related
to printk. Since console_lock() can sleep, any caller of console_lock()
_should_ be allowed to perform the cond_resched(). (The printk thread is
an exception here!)

Here is one call chain I picked out:

tty_io.c:tty_write_message()
    mutex_lock()
    tty->ops->write() -> usb-serial.c:serial_write()
        port->serial->type->write() -> vt.c:con_write()
            do_con_write()
                console_lock()
                    console_may_schedule = 1;
                console_conditional_schedule();
                    cond_resched();

From the mutex_lock() we can see that we are in a non-atomic context. In
this case it is OK to call console_lock() and cond_resched().

John Ogness