On 2020-04-15, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> 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