Re: [PATCH] printk: fix one circular lockdep warning about console_lock

[Jan Kara]

On Tue 11-02-14 13:19:27, Andrew Morton wrote:
> On Tue, 11 Feb 2014 14:50:00 +0800 <jiel@xxxxxxxxxxx> wrote:
> 
> > From: Jane Li <jiel@xxxxxxxxxxx>
> > 
> > This patch tries to fix a warning about possible circular locking
> > dependency.
> > 
> > If do in following sequence:
> >     enter suspend ->  resume ->  plug-out CPUx (echo 0 > cpux/online)
> > lockdep will show warning as following:
> > 
> > ======================================================
> > [ INFO: possible circular locking dependency detected ]
> > 3.10.0 #2 Tainted: G           O
> > -------------------------------------------------------
> > sh/1271 is trying to acquire lock:
> > (console_lock){+.+.+.}, at: [<c06ebf7c>] console_cpu_notify+0x20/0x2c
> > but task is already holding lock:
> > (cpu_hotplug.lock){+.+.+.}, at: [<c012b4e8>] cpu_hotplug_begin+0x2c/0x58
> > which lock already depends on the new lock.
> > 
> > the existing dependency chain (in reverse order) is:
> > -> #2 (cpu_hotplug.lock){+.+.+.}:
> > [<c017bb7c>] lock_acquire+0x98/0x12c
> > [<c06f5014>] mutex_lock_nested+0x50/0x3d8
> > [<c012b4e8>] cpu_hotplug_begin+0x2c/0x58
> > [<c06ebfac>] _cpu_up+0x24/0x154
> > [<c06ec140>] cpu_up+0x64/0x84
> > [<c0981834>] smp_init+0x9c/0xd4
> > [<c0973880>] kernel_init_freeable+0x78/0x1c8
> > [<c06e7f40>] kernel_init+0x8/0xe4
> > [<c010eec8>] ret_from_fork+0x14/0x2c
> > 
> > -> #1 (cpu_add_remove_lock){+.+.+.}:
> > [<c017bb7c>] lock_acquire+0x98/0x12c
> > [<c06f5014>] mutex_lock_nested+0x50/0x3d8
> > [<c012b758>] disable_nonboot_cpus+0x8/0xe8
> > [<c016b83c>] suspend_devices_and_enter+0x214/0x448
> > [<c016bc54>] pm_suspend+0x1e4/0x284
> > [<c016bdcc>] try_to_suspend+0xa4/0xbc
> > [<c0143848>] process_one_work+0x1c4/0x4fc
> > [<c0143f80>] worker_thread+0x138/0x37c
> > [<c014aaf8>] kthread+0xa4/0xb0
> > [<c010eec8>] ret_from_fork+0x14/0x2c
> > 
> > -> #0 (console_lock){+.+.+.}:
> > [<c017b5d0>] __lock_acquire+0x1b38/0x1b80
> > [<c017bb7c>] lock_acquire+0x98/0x12c
> > [<c01288c4>] console_lock+0x54/0x68
> > [<c06ebf7c>] console_cpu_notify+0x20/0x2c
> > [<c01501d4>] notifier_call_chain+0x44/0x84
> > [<c012b448>] __cpu_notify+0x2c/0x48
> > [<c012b5b0>] cpu_notify_nofail+0x8/0x14
> > [<c06e81bc>] _cpu_down+0xf4/0x258
> > [<c06e8344>] cpu_down+0x24/0x40
> > [<c06e921c>] store_online+0x30/0x74
> > [<c03b7298>] dev_attr_store+0x18/0x24
> > [<c025fc5c>] sysfs_write_file+0x16c/0x19c
> > [<c0207a98>] vfs_write+0xb4/0x190
> > [<c0207e58>] SyS_write+0x3c/0x70
> > [<c010ee00>] ret_fast_syscall+0x0/0x48
> > 
> > Chain exists of:
> >    console_lock --> cpu_add_remove_lock --> cpu_hotplug.lock
> > 
> > Possible unsafe locking scenario:
> >        CPU0                    CPU1
> >        ----                    ----
> > lock(cpu_hotplug.lock);
> >                                lock(cpu_add_remove_lock);
> >                                lock(cpu_hotplug.lock);
> > lock(console_lock);
> >   *** DEADLOCK ***
> 
> These traces hurt my brain.
> 
> > There are three locks involved in two sequence:
> > a) pm suspend:
> > 	console_lock (@suspend_console())
> > 	cpu_add_remove_lock (@disable_nonboot_cpus())
> > 	cpu_hotplug.lock (@_cpu_down())
> 
> But but but.  suspend_console() releases console_sem again.  So the
> sequence is actually
> 
>  	down(&console_sem) (@suspend_console())
>  	up(&console_sem) (@suspend_console())
>  	cpu_add_remove_lock (@disable_nonboot_cpus())
>  	cpu_hotplug.lock (@_cpu_down())
> 
> So console_sem *doesn't* nest outside cpu_add_remove_lock and
> cpu_hotplug.lock.
  Exactly. My take would be that the lockdep annotation of console_sem is
just missing
  mutex_release(&console_lock_dep_map, 1, _RET_IP_);
in suspend_console() and similar counterpart in resume_console(). We are
doing the annotation by hand and apparently this got missed.

...
> > +void console_flush(void)
> > +{
> > +	if (console_trylock())
> > +		console_unlock();
> > +}
> > +
> > +/**
> >   * console_cpu_notify - print deferred console messages after CPU hotplug
> >   * @self: notifier struct
> >   * @action: CPU hotplug event
> > @@ -1911,8 +1925,7 @@ static int console_cpu_notify(struct notifier_block *self,
> >  	case CPU_DEAD:
> >  	case CPU_DOWN_FAILED:
> >  	case CPU_UP_CANCELED:
> > -		console_lock();
> > -		console_unlock();
> > +		console_flush();
> >  	}
> >  	return NOTIFY_OK;
> 
> Well, this is a bit hacky and makes the already-far-too-complex code
> even more complex.  If it is indeed the case that the deadlock cannot
> really occur then let's try to find a way of suppressing the lockdep
> warning without making runtime changes.
> 
> What I'm struggling with is what *should* the ranking of these locks be?
> From a conceptual high-level design standpoint, which is the
> "innermost" lock?  I tend to think that it is console_lock, because
> blocking CPU hotplug is a quite high-level operation.
> 
> But console_lock is such a kooky special-case in the way it is used to
> control the printk corking that it is hard to take general rules and
> apply them here.
  Currently I think it should be pretty much the innermost lock for
anything except for console driver special locks.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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