Re: printk deadlock due to double lock attempt on current CPU's runqueue

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 09, 2021 at 12:06:48PM -0800, Sultan Alsawaf wrote:
> Hi,
> 
> I encountered a printk deadlock on 5.13 which appears to still affect the latest
> kernel. The deadlock occurs due to printk being used while having the current
> CPU's runqueue locked, and the underlying framebuffer console attempting to lock
> the same runqueue when printk tries to flush the log buffer.

Yes, that's a known 'feature' of some consoles. printk() is in the
process of being reworked to not call con->write() from the printk()
calling context, which would go a long way towards fixing this.

>   #27 [ffffc900005b8e28] enqueue_task_fair at ffffffff8129774a  <-- SCHED_WARN_ON(rq->tmp_alone_branch != &rq->leaf_cfs_rq_list);
>   #28 [ffffc900005b8ec0] activate_task at ffffffff8125625d
>   #29 [ffffc900005b8ef0] ttwu_do_activate at ffffffff81257943
>   #30 [ffffc900005b8f28] sched_ttwu_pending at ffffffff8125c71f <-- locks this CPU's runqueue
>   #31 [ffffc900005b8fa0] flush_smp_call_function_queue at ffffffff813c6833
>   #32 [ffffc900005b8fd8] generic_smp_call_function_single_interrupt at ffffffff813c7f58
>   #33 [ffffc900005b8fe0] __sysvec_call_function_single at ffffffff810f1456
>   #34 [ffffc900005b8ff0] sysvec_call_function_single at ffffffff831ec1bc
>   --- <IRQ stack> ---
>   #35 [ffffc9000019fda8] sysvec_call_function_single at ffffffff831ec1bc
>       RIP: ffffffff831ed06e  RSP: ffffed10438a6a49  RFLAGS: 00000001
>       RAX: ffff888100d832c0  RBX: 0000000000000000  RCX: 1ffff92000033fd7
>       RDX: 0000000000000000  RSI: ffff888100d832c0  RDI: ffffed10438a6a49
>       RBP: ffffffff831ec166   R8: dffffc0000000000   R9: 0000000000000000
>       R10: ffffffff83400e22  R11: 0000000000000000  R12: ffffffff831ed83e
>       R13: 0000000000000000  R14: ffffc9000019fde8  R15: ffffffff814d4d9d
>       ORIG_RAX: ffff88821c53524b  CS: 0001  SS: ef073a2
>   WARNING: possibly bogus exception frame
> ----------------------->8-----------------------
> 
> The catalyst is that CONFIG_SCHED_DEBUG is enabled and the tmp_alone_branch
> assertion fails (Peter, is this bad?).

Yes, that's not good. IIRC Vincent and Michal were looking at that code
recently.

> I'm not sure what the *correct* solution is here (don't use printk while having
> a runqueue locked? don't use schedule_work() from the fbcon path? tell printk
> to use one of its lock-less backends?), so I've cc'd all the relevant folks.

I'm a firm believer in early_printk serial consoles.