Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for device online store callbacks

[Li Zhong <zhong@linux.vnet.ibm.com>]

On Thu, 2014-04-24 at 12:02 +0200, Rafael J. Wysocki wrote:
> On 4/24/2014 10:59 AM, Li Zhong wrote:
> > On Wed, 2014-04-23 at 18:12 +0200, Rafael J. Wysocki wrote:
> >> On 4/23/2014 4:23 PM, Tejun Heo wrote:
> >>> Hello, Rafael.
> >> Hi,
> >>
> >>> On Wed, Apr 23, 2014 at 12:21:33AM +0200, Rafael J. Wysocki wrote:
> >>>> Can you please elaborate a bit?
> >>> Because it can get involved in larger locking dependency issues by
> >>> joining dependency graphs of two otherwise largely disjoint
> >>> subsystems.  It has potential to create possible deadlocks which don't
> >>> need to exist.
> >> Well, I do my best not to add unnecessary locks if that's what you mean.
> >>
> >>>> It is there to protect hotplug operations involving multiple devices
> >>>> (in different subsystems) from racing with each other.  Why exactly
> >>>> is it bad?
> >>> But why would different subsystems, say cpu and memory, use the same
> >>> lock?  Wouldn't those subsystems already have proper locking inside
> >>> their own subsystems?
> >> That locking is not sufficient.
> >>
> >>> Why add this additional global lock across multiple subsystems?
> >> That basically is because of how eject works when it is triggered via ACPI.
> >>
> >> It is signaled for a device at the top of a subtree.  It may be a
> >> container of some sort and the eject involves everything below that
> >> device in the ACPI namespace.  That may involve multiple subsystem
> >> (CPUs, memory, PCI host bridge, etc.).
> >>
> >> We do that in two steps, offline (which can fail) and eject proper
> >> (which can't fail and makes all of the involved devices go away). All
> >> that has to be done in one go with respect to the sysfs-triggered
> >> offline/online and that's why the lock is there.
> > Thank you for the education. It's more clear to me now why we need this
> > lock.
> >
> > I still have some small questions about when this lock is needed:
> >
> > I could understand sysfs-triggered online is not acceptable when
> > removing devices in multiple subsystems. But maybe concurrent offline
> > and remove(with proper per subsystem locks) seems not harmful?
> >
> > And if we just want to remove some devices in a specific subsystem, e.g.
> > like writing /cpu/release, if it just wants to offline and remove some
> > cpus, then maybe we don't require the device_hotplug_lock to be taken?
> 
> No and no.
> 
> If the offline phase fails for any device in the subtree, we roll back 
> the operation
> and online devices that have already been offlined by it.  Also the ACPI 
> hot-addition
> needs to acquire device_hotplug_lock so that it doesn't race with ejects 
> and so
> that lock needs to be taken by sysfs-triggered offline too.

I can understand that hot-addition needs the device_hotplug lock, but
still not very clear about the offline. 

I guess your are describing following scenario: 

user A: (trying remove cpu 1 and memory 1-10)

lock_device_hotplug
offline cpu with cpu locks          -- successful
offline memories with memory locks  -- failed, e.g. for memory8
online cpu and memory with their locks
unlock_device_hotplug

user B: (trying offline cpu 1)

offline cpu with cpu locks

But I don't see any problem for user B not taking the device_hotplug
lock. The result may be different for user B to take or not take the
lock. But I think it could be seen as concurrent online/offline for cpu1
under cpu hotplug locks, which just depends on which is executed last? 

Or did I miss something here? 

Thanks, Zhong

> Thanks,
> Rafael
>