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

[Michal Hocko]

On Wed 09-10-19 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/
> > > 
> > > [  297.425964] -> #1 (&port_lock_key){-.-.}:
> > > [  297.425967]        __lock_acquire+0x5b3/0xb40
> > > [  297.425967]        lock_acquire+0x126/0x280
> > > [  297.425968]        _raw_spin_lock_irqsave+0x3a/0x50
> > > [  297.425969]        serial8250_console_write+0x3e4/0x450
> > > [  297.425970]        univ8250_console_write+0x4b/0x60
> > > [  297.425970]        console_unlock+0x501/0x750
> > > [  297.425971]        vprintk_emit+0x10d/0x340
> > > [  297.425972]        vprintk_default+0x1f/0x30
> > > [  297.425972]        vprintk_func+0x44/0xd4
> > > [  297.425973]        printk+0x9f/0xc5
> > > [  297.425974]        register_console+0x39c/0x520
> > > [  297.425975]        univ8250_console_init+0x23/0x2d
> > > [  297.425975]        console_init+0x338/0x4cd
> > > [  297.425976]        start_kernel+0x534/0x724
> > > [  297.425977]        x86_64_start_reservations+0x24/0x26
> > > [  297.425977]        x86_64_start_kernel+0xf4/0xfb
> > > [  297.425978]        secondary_startup_64+0xb6/0xc0
> > > 
> > > where the 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

Can you paste the full lock chain graph to be sure we are on the same
page?

-- 
Michal Hocko
SUSE Labs