Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

[Qian Cai]

On Wed, 2019-10-09 at 16:24 +0200, Petr Mladek wrote:
> On Wed 2019-10-09 09:43:13, Qian Cai wrote:
> > On Wed, 2019-10-09 at 15:27 +0200, Michal Hocko wrote:
> > > On Wed 09-10-19 09:06:42, Qian Cai wrote:
> > > [...]
> > > > https://lore.kernel.org/linux-mm/1570460350.5576.290.camel@xxxxxx/
> > > > 
> > > > [ÂÂ297.425964] -> #1 (&port_lock_key){-.-.}:
> > > > [ÂÂ297.425967]ÂÂÂÂÂÂÂÂ__lock_acquire+0x5b3/0xb40
> > > > [ÂÂ297.425967]ÂÂÂÂÂÂÂÂlock_acquire+0x126/0x280
> > > > [ÂÂ297.425968]ÂÂÂÂÂÂÂÂ_raw_spin_lock_irqsave+0x3a/0x50
> > > > [ÂÂ297.425969]ÂÂÂÂÂÂÂÂserial8250_console_write+0x3e4/0x450
> > > > [ÂÂ297.425970]ÂÂÂÂÂÂÂÂuniv8250_console_write+0x4b/0x60
> > > > [ÂÂ297.425970]ÂÂÂÂÂÂÂÂconsole_unlock+0x501/0x750
> > > > [ÂÂ297.425971]ÂÂÂÂÂÂÂÂvprintk_emit+0x10d/0x340
> > > > [ÂÂ297.425972]ÂÂÂÂÂÂÂÂvprintk_default+0x1f/0x30
> > > > [ÂÂ297.425972]ÂÂÂÂÂÂÂÂvprintk_func+0x44/0xd4
> > > > [ÂÂ297.425973]ÂÂÂÂÂÂÂÂprintk+0x9f/0xc5
> > > > [ÂÂ297.425974]ÂÂÂÂÂÂÂÂregister_console+0x39c/0x520
> > > > [ÂÂ297.425975]ÂÂÂÂÂÂÂÂuniv8250_console_init+0x23/0x2d
> > > > [ÂÂ297.425975]ÂÂÂÂÂÂÂÂconsole_init+0x338/0x4cd
> > > > [ÂÂ297.425976]ÂÂÂÂÂÂÂÂstart_kernel+0x534/0x724
> > > > [ÂÂ297.425977]ÂÂÂÂÂÂÂÂx86_64_start_reservations+0x24/0x26
> > > > [ÂÂ297.425977]ÂÂÂÂÂÂÂÂx86_64_start_kernel+0xf4/0xfb
> > > > [ÂÂ297.425978]ÂÂÂÂÂÂÂÂsecondary_startup_64+0xb6/0xc0
> > > > 
> > > > whereÂthe report again show the early boot call trace for the locking
> > > > dependency,
> > > > 
> > > > console_owner --> port_lock_key
> > > > 
> > > > but that dependency clearly not only happen in the early boot.
> > > 
> > > Can you provide an example of the runtime dependency without any early
> > > boot artifacts? Because this discussion really doens't make much sense
> > > without a clear example of a _real_ lockdep report that is not a false
> > > possitive. All of them so far have been concluded to be false possitive
> > > AFAIU.
> > 
> > An obvious one is in the above link. Just replace the trace in #1 above with
> > printk() from anywhere, i.e., just ignore the early boot calls there as they are
> >  not important.
> > 
> > printk()
> >   console_unlock()
> >     console_lock_spinning_enable() --> console_owner_lock
> >   call_console_drivers()
> >     serial8250_console_write() --> port->lock
> 
> Please, find the location where this really happens and then suggests
> how the real deadlock could get fixed. So far, we have seen only
> false positives and theoretical scenarios.

Now the bar is higher again. You are now asking me to actually trigger this
potential deadlock live. I am probably better off buying some lottery tickets
then if I could be that lucky.