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

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 09, 2021 at 10:38:47PM +0100, Peter Zijlstra wrote:
> 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.

I'm a bit out of the loop but from lwn articles my understanding is that
part of upstreaming from -rt we no longer have the explicit "I'm a safe
console for direct printing" opt-in. Which I get from a backwards compat
pov, but I still think for at least fbcon we really should never attempt a
direct printk con->write, it's just all around terrible.

And it's getting worse by the year:
- direct scanout displays (i.e. just a few mmio writes and it will show
  up) are on the way out at least in laptops, everyone gets self-refresh
  (dp psr) under software control, so without being able to kick a kthread
  off nothing shows up except more oopses

- because of the impendence mismatch between fbdev and drm-kms we even go
  ever more this direction for dumb framebuffers, including the firmware
  boot-up framebuffer simpledrm. This could perhaps be fixed with a new
  dedicate console driver directly on top of drm-kms, but that's on the
  wishlist for years and I don't see anyone typing that.

So yeah for fbcon at least I think we really should throw out direct
con->write from printk completely.

Also adding John Ogness.
-Daniel

> 
> >   #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.

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch