[PATCH] RFC: console: hack up console_trylock more

[Daniel Vetter]

On Fri, May 3, 2019 at 5:14 PM Petr Mladek <pmladek at suse.com> wrote:
> On Thu 2019-05-02 16:16:43, Daniel Vetter wrote:
> > console_trylock, called from within printk, can be called from pretty
> > much anywhere. Including try_to_wake_up. Note that this isn't common,
> > usually the box is in pretty bad shape at that point already. But it
> > really doesn't help when then lockdep jumps in and spams the logs,
> > potentially obscuring the real backtrace we're really interested in.
> > One case I've seen (slightly simplified backtrace):
> >
> >  Call Trace:
> >   <IRQ>
> >   console_trylock+0xe/0x60
> >   vprintk_emit+0xf1/0x320
> >   printk+0x4d/0x69
> >   __warn_printk+0x46/0x90
> >   native_smp_send_reschedule+0x2f/0x40
> >   check_preempt_curr+0x81/0xa0
> >   ttwu_do_wakeup+0x14/0x220
> >   try_to_wake_up+0x218/0x5f0
> >   pollwake+0x6f/0x90
> >   credit_entropy_bits+0x204/0x310
> >   add_interrupt_randomness+0x18f/0x210
> >   handle_irq+0x67/0x160
> >   do_IRQ+0x5e/0x130
> >   common_interrupt+0xf/0xf
> >   </IRQ>
> >
> > This alone isn't a problem, but the spinlock in the semaphore is also
> > still held while waking up waiters (up() -> __up() -> try_to_wake_up()
> > callchain), which then closes the runqueue vs. semaphore.lock loop,
> > and upsets lockdep, which issues a circular locking splat to dmesg.
> > Worse it upsets developers, since we don't want to spam dmesg with
> > clutter when the machine is dying already.
> >
> > Fix this by creating a __down_trylock which only trylocks the
> > semaphore.lock. This isn't correct in full generality, but good enough
> > for console_lock:
> >
> > - there's only ever one console_lock holder, we won't fail spuriously
> >   because someone is doing a down() or up() while there's still room
> >   (unlike other semaphores with count > 1).
> >
> > - console_unlock() has one massive retry loop, which will catch anyone
> >   who races the trylock against the up(). This makes sure that no
> >   printk lines will get lost. Making the trylock more racy therefore
> >   has no further impact.
>
> To be honest, I do not see how this could solve the problem.
>
> The circular dependency is still there. If the new __down_trylock()
> succeeds then console_unlock() will get called in the same context
> and it will still need to call up() -> try_to_wake_up().
>
> Note that there are many other console_lock() callers that might
> happen in parallel and might appear in the wait queue.

Hm right. It's very rare we hit this in our CI and I don't know how to
repro otherwise, so just threw this out at the wall to see if it
sticks. I'll try and come up with a new trick then.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch