On Wed, Mar 18, 2020 at 12:33:12PM +0100, Lukas Wunner wrote:
> 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 54ecb8f7028c ("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 6c35a1ac3da6 ("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 3943af9d01e9 ("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: 54ecb8f7028c ("PCI: pciehp: Avoid returning prematurely from sysfs requests")
> Reported-by: David Hoyer <David.Hoyer@xxxxxxxxxx>
> Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v4.19+
> Cc: Keith Busch <kbusch@xxxxxxxxxx>
Applied to pci/hotplug for v5.7, thanks!
> ---
> drivers/pci/hotplug/pciehp_hpc.c | 14 +++++++-------
> 1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
> index e4627c68b30f..5f1a27bfcb19 100644
> --- a/drivers/pci/hotplug/pciehp_hpc.c
> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -663,17 +663,15 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
> if (atomic_fetch_and(~RERUN_ISR, &ctrl->pending_events) & RERUN_ISR) {
> ret = pciehp_isr(irq, dev_id);
> enable_irq(irq);
> - if (ret != IRQ_WAKE_THREAD) {
> - pci_config_pm_runtime_put(pdev);
> - return ret;
> - }
> + if (ret != IRQ_WAKE_THREAD)
> + goto out;
> }
>
> synchronize_hardirq(irq);
> events = atomic_xchg(&ctrl->pending_events, 0);
> if (!events) {
> - pci_config_pm_runtime_put(pdev);
> - return IRQ_NONE;
> + ret = IRQ_NONE;
> + goto out;
> }
>
> /* Check Attention Button Pressed */
> @@ -702,10 +700,12 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
> pciehp_handle_presence_or_link_change(ctrl, events);
> up_read(&ctrl->reset_lock);
>
> + ret = IRQ_HANDLED;
> +out:
> pci_config_pm_runtime_put(pdev);
> ctrl->ist_running = false;
> wake_up(&ctrl->requester);
> - return IRQ_HANDLED;
> + return ret;
> }
>
> static int pciehp_poll(void *data)
> --
> 2.25.0
>
