Re: printk, vt: sleep from invalid context bug

[Petr Mladek]

On Tue 2026-02-17 09:14:12, Nam Cao wrote:
> Hi,
> 
> Since commit 1bc9a28f076f ("printk: Use console_flush_one_record for
> legacy printer kthread"), I see this warning:
> 
> BUG: sleeping function called from invalid context at kernel/printk/printk.c:3431
> in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 14, name: pr/legacy
> preempt_count: 0, expected: 0
> RCU nest depth: 1, expected: 0
> 4 locks held by pr/legacy/14:
>  #0: ffffffff807dcdf0 (console_lock){+.+.}-{0:0}, at: legacy_kthread_func+0x40/0x114
>  #1: ffffffff807dce30 (console_srcu){....}-{0:0}, at: console_flush_one_record+0x0/0x3c8
>  #2: ffffffff8083af88 (printing_lock){+.+.}-{3:3}, at: vt_console_print+0x48/0x330
>  #3: ffffffff807df2e8 (rcu_read_lock){....}-{1:3}, at: rt_spin_trylock+0x24/0x138
> CPU: 0 UID: 0 PID: 14 Comm: pr/legacy Not tainted 6.19.0-rvvm-09292-g7449f86bafcd #79 PREEMPT_RT 
> Hardware name: RVVM v0.7-git-g8c45ccf (DT)
> Call Trace:
>     walk_stackframe+0x0/0x80
>     dump_stack_lvl+0x4e/0x74
>     rt_spin_trylock+0x9e/0x138
>     __might_resched+0x19e/0x1e0
>     fbcon_redraw+0x74/0x1a0
>     fbcon_scroll+0xf2/0x13c
>     con_scroll+0x122/0x188
>     lf+0x6a/0x74
>     vt_console_print+0x2ac/0x330
>     console_flush_one_record+0x208/0x3c8
>     console_flush_one_record+0x3c4/0x3c8
>     kthread+0xc6/0x100
>     legacy_kthread_func+0x4c/0x114
>     prio_changed_stop+0xc/0x10
>     kthread+0xc6/0x100
>     ret_from_fork_kernel_asm+0x12/0x18
>     ret_from_fork_kernel+0xe/0x3fc
>     rt_spin_unlock+0x56/0x128
>     kthread_affine_node+0x88/0x8c
>     ret_from_fork_kernel_asm+0x12/0x18
> 
> If I understand it correctly, vt_console_print() grabs the spin lock
> "printing_lock", and then calls fbcon_redraw() which does
> console_conditional_schedule(), triggering the warning.

Sebastian has recently sent a patch which should remove the
cond_resched(), see
https://lore.kernel.org/all/20260126180836.SNCdMW2f@xxxxxxxxxxxxx

It seems that the patch has made it upstream in this merge window
for 7.0, see the commit 8e9bf8b9e8c0 ("printk, vt, fbcon: Remove
console_conditional_schedule()").

Best Regards,
Petr