Date:	Wed, 18 Mar 2015 03:04:07 -0700
From:	Guenter Roeck <groeck@...iper.net>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Rajat Jain <rajatxjain@...il.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rajat Jain <rajatjain@...iper.net>, <rthirumal@...iper.net>,
	<sanjayj@...iper.net>, Rafael Wysocki <rjw@...ysocki.net>
Subject: Re: Hit a deadlock: between AER and pcieport/pciehp

On Tue, Mar 17, 2015 at 09:28:24PM -0500, Bjorn Helgaas wrote:
> [+cc Rafael]
> 
> Hi Rajat,
> 
> On Tue, Mar 17, 2015 at 2:11 PM, Rajat Jain <rajatxjain@...il.com> wrote:
> > Hello,
> >
> > I was wondering if any one has a any suggestions to make here. I
> > believe this is a pretty serious deadlock - and I'm looking for ideas
> > on what should be the right way to fix this.
> 
> I agree, this definitely sounds like a real problem.  I'm not ignoring
> it; I just haven't had time to look into it :)
> 
> After ten seconds of thought, my suggestion is to try to make this
> work in a way that doesn't require taking the mutexes in two different
> orders.  It might be *possible* to write code that is smart enough to
> take them in different orders, but I'm pretty sure our automated lock
> checking tools wouldn't be that smart.
> 

Assuming that pci_walk_bus needs to hold pci_bus_sem, and pci_create_slot
needs to hold it as well, that may be tricky. More likely one just must
not call device_lock() from any function called through pci_walk_bus.

Does report_error_detected() really need to call device_lock(),
or would get_device() be sufficient ? Or even nothing ?

Guenter

> I added Rafael because he recently did some work on PCI bus locking
> and might have better ideas than I do.
> 
> > On Wed, Mar 11, 2015 at 6:48 PM, Rajat Jain <rajatxjain@...il.com> wrote:
> >> Hello,
> >>
> >>
> >> I have hit a kernel deadlock situation on my system that has
> >> hierarchical hot plug situations (i.e. we can hot-plug a card, that
> >> itself may have a hot-plug slot for another level of hot-pluggable
> >> add-on cards). In summary, I see 2 threads that are both waiting on
> >> mutexes that is acquired by the other one. The mutexes are the
> >> (global) "pci_bus_sem" and "device->mutex" respectively.
> >>
> >>
> >> Thread1
> >>  =======
> >>  This is the pciehp worker thread, that scans a new card, and on
> >> finding that there is a hotplug slot downstream, tries to
> >> pci_create_slot().
> >>  pciehp_power_thread()
> >>    -> pciehp_enable_slot()
> >>      -> pciehp_configure_device()
> >>        -> pci_bus_add_devices() discovers all devices including a new
> >> hotplug slot.
> >>          -> ....(etc)...
> >>          -> device_attach(dev) (for the newly discovered HP slot /
> >> downstream port)
> >>            -> device_lock(dev) SUCCESSFULLY ACQUIRES dev->mutex for
> >> the new slot.
> >>          -> ....(etc)...
> >>          -> ... (goes on)
> >>          -> pciehp_probe(dev)
> >>              -> __pci_hp_register()
> >>                 -> pci_create_slot()
> >>                      -> down_write(pci_bus_sem); /* Deadlocked */
> >>
> >>  This how the stack looks like:
> >>   [<ffffffff814e9923>] call_rwsem_down_write_failed+0x13/0x20
> >>  [<ffffffff81522d4f>] pci_create_slot+0x3f/0x280
> >>  [<ffffffff8152c030>] __pci_hp_register+0x70/0x400
> >>  [<ffffffff8152cf49>] pciehp_probe+0x1a9/0x450
> >>  [<ffffffff8152865d>] pcie_port_probe_service+0x3d/0x90
> >>  [<ffffffff815c45b9>] driver_probe_device+0xf9/0x350
> >>  [<ffffffff815c490b>] __device_attach+0x4b/0x60
> >>  [<ffffffff815c25a6>] bus_for_each_drv+0x56/0xa0
> >>  [<ffffffff815c4468>] device_attach+0xa8/0xc0
> >>  [<ffffffff815c38d0>] bus_probe_device+0xb0/0xe0
> >>  [<ffffffff815c16ce>] device_add+0x3de/0x560
> >>  [<ffffffff815c1a2e>] device_register+0x1e/0x30
> >>  [<ffffffff81528aef>] pcie_port_device_register+0x32f/0x510
> >>  [<ffffffff81528eb8>] pcie_portdrv_probe+0x48/0x80
> >>  [<ffffffff8151b17c>] pci_device_probe+0x9c/0xf0
> >>  [<ffffffff815c45b9>] driver_probe_device+0xf9/0x350
> >>  [<ffffffff815c490b>] __device_attach+0x4b/0x60
> >>  [<ffffffff815c25a6>] bus_for_each_drv+0x56/0xa0
> >>  [<ffffffff815c4468>] device_attach+0xa8/0xc0
> >>  [<ffffffff815116c1>] pci_bus_add_device+0x41/0x70
> >>  [<ffffffff81511a41>] pci_bus_add_devices+0x41/0x90
> >>  [<ffffffff81511a6f>] pci_bus_add_devices+0x6f/0x90
> >>  [<ffffffff8152e7e2>] pciehp_configure_device+0xa2/0x140
> >>  [<ffffffff8152df08>] pciehp_enable_slot+0x188/0x2d0
> >>  [<ffffffff8152e3d1>] pciehp_power_thread+0x2b1/0x3c0
> >>  [<ffffffff810d92a0>] process_one_work+0x1d0/0x510
> >>  [<ffffffff810d9cc1>] worker_thread+0x121/0x440
> >>  [<ffffffff810df0bf>] kthread+0xef/0x110
> >>  [<ffffffff81a4d8ac>] ret_from_fork+0x7c/0xb0
> >>  [<ffffffffffffffff>] 0xffffffffffffffff
> >>
> >>
> >>  Thread2
> >>  =======
> >>  While the above thread is doing its work, the root port gets a
> >> completion timeout. And thus the AER Error recovery worker thread
> >> kicks in to handle that error. And as part of that error recovery -
> >> since the completion timeout was detected at root port, attempts to
> >> see for ALL the devices downstream if they have an error handler that
> >> need to be called. Here is what happens:
> >>
> >>
> >> aer_isr()
> >>    -> aer_isr_one_error()
> >>      -> aer_process_err_device()
> >>         -> ... (etc)...
> >>           -> do_recovery()
> >>             -> broadcast_error_message()
> >>               -> pci_walk_bus( ..., report_error_detected,...) /*
> >> effectively for all buses below root port */
> >>                     -> down_read(&pci_bus_sem);  /* SUCCESSFULLY
> >> ACQUIRES the semaophore */
> >>                     -> report_error_detected(dev) /* for the newly
> >> detected slot */
> >>                          -> device_lock(dev) /* Deadlocked */
> >>
> >>  This is how the stack looks like:
> >>  [<ffffffff81529e7e>] report_error_detected+0x4e/0x170 <--- Waiting on
> >> device_lock()
> >>  [<ffffffff8151162e>] pci_walk_bus+0x4e/0xa0
> >>  [<ffffffff81529b84>] broadcast_error_message+0xc4/0xf0
> >>  [<ffffffff81529bed>] do_recovery+0x3d/0x280
> >>  [<ffffffff8152a5d0>] aer_isr+0x300/0x3e0
> >>  [<ffffffff810d92a0>] process_one_work+0x1d0/0x510
> >>  [<ffffffff810d9cc1>] worker_thread+0x121/0x440
> >>  [<ffffffff810df0bf>] kthread+0xef/0x110
> >>  [<ffffffff81a4d8ac>] ret_from_fork+0x7c/0xb0
> >>  [<ffffffffffffffff>] 0xffffffffffffffff
> >>
> >>
> >> As a temporary work around to let me proceed, I was thinking may be I
> >> could change in report_error_detected() such that completion timeouts
> >> errors may not be broadcast (do we really have any drivers that have
> >> aer handlers that handle such an error? What would the handler do
> >> anyway to fix such an error?)
> >>
> >>
> >> But not sure what the right solution might look like. I thought about
> >> whether these locks should have been taken in a particular order in
> >> order to avoid this problem, but looking at the stack there seems to
> >> be no other way. What do you think is the best way to fix this
> >> deadlock?
> >>
> >> Any help or suggestions in this regard are greatly appreciated.
> >>
> >>  Thanks,
> >>
> >> Rajat
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