David Hoyer reports that powering pciehp slots up or down via sysfs may
hang:  The call to wait_event() in pciehp_sysfs_enable_slot() and
_disable_slot() does not return because ctrl->ist_running remains true.

This flag, which was introduced by commit 157c1062 ("PCI: pciehp: Avoid
returning prematurely from sysfs requests"), signifies that the IRQ thread
pciehp_ist() is running.  It is set to true at the top of pciehp_ist() and
reset to false at the end.  However there are two additional return
statements in pciehp_ist() before which the commit neglected to reset the
flag to false and wake up waiters for the flag.

That omission opens up the following race when powering up the slot:

* pciehp_ist() runs because a PCI_EXP_SLTSTA_PDC event was requested
  by pciehp_sysfs_enable_slot()

* pciehp_ist() turns on slot power via the following call stack:
  pciehp_handle_presence_or_link_change() -> pciehp_enable_slot() ->
  __pciehp_enable_slot() -> board_added() -> pciehp_power_on_slot()

* after slot power is turned on, the link comes up, resulting in a
  PCI_EXP_SLTSTA_DLLSC event

* the IRQ handler pciehp_isr() stores the event in ctrl->pending_events
  and returns IRQ_WAKE_THREAD

* the IRQ thread is already woken (it's bringing up the slot), but the
  genirq code remembers to re-run the IRQ thread after it has finished
  (such that it can deal with the new event) by setting IRQTF_RUNTHREAD
  via __handle_irq_event_percpu() -> __irq_wake_thread()

* the IRQ thread removes PCI_EXP_SLTSTA_DLLSC from ctrl->pending_events
  via board_added() -> pciehp_check_link_status() in order to deal with
  presence and link flaps per commit 6c35a1ac ("PCI: pciehp:
  Tolerate initially unstable link")

* after pciehp_ist() has successfully brought up the slot, it resets
  ctrl->ist_running to false and wakes up the sysfs requester

* the genirq code re-runs pciehp_ist(), which sets ctrl->ist_running
  to true but then returns with IRQ_NONE because ctrl->pending_events
  is empty

* pciehp_sysfs_enable_slot() is finally woken but notices that
  ctrl->ist_running is true, hence continues waiting

The only way to get the hung task going again is to trigger a hotplug
event which brings down the slot, e.g. by yanking out the card.

The same race exists when powering down the slot because remove_board()
likewise clears link or presence changes in ctrl->pending_events per commit
3943af9d ("PCI: pciehp: Ignore Link State Changes after powering off a
slot") and thereby may cause a re-run of pciehp_ist() which returns with
IRQ_NONE without resetting ctrl->ist_running to false.

Fix by adding a goto label before the teardown steps at the end of
pciehp_ist() and jumping to that label from the two return statements which
currently neglect to reset the ctrl->ist_running flag.

Fixes: 157c1062 ("PCI: pciehp: Avoid returning prematurely from sysfs requests")
Link: https://lore.kernel.org/r/cca1effa488065cb055120aa01b65719094bdcb5.1584530321.git.lukas@wunner.deReported-by: David Hoyer <David.Hoyer@netapp.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Cc: stable@vger.kernel.org	# v4.19+