RE: [PATCH 2/3] PCI: hv: serialize the present/eject work items

[Michael Kelley (EOSG)]

> -----Original Message-----
> From: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-owner@xxxxxxxxxxxxxxx> On Behalf
> Of Dexuan Cui
> Sent: Friday, March 2, 2018 4:21 PM
> To: bhelgaas@xxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>;
> Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; jasowang@xxxxxxxxxx;
> vkuznets@xxxxxxxxxx; marcelo.cerri@xxxxxxxxxxxxx; Dexuan Cui <decui@xxxxxxxxxxxxx>;
> Jack Morgenstein <jackm@xxxxxxxxxxxx>; stable@xxxxxxxxxxxxxxx
> Subject: [PATCH 2/3] PCI: hv: serialize the present/eject work items
> 
> When we hot-remove the device, we first receive a PCI_EJECT message and
> then receive a PCI_BUS_RELATIONS message with bus_rel->device_count == 0.
> 
> The first message is offloaded to hv_eject_device_work(), and the second
> is offloaded to pci_devices_present_work(). Both the paths can be running
> list_del(&hpdev->list_entry), causing general protection fault, because
> system_wq can run them concurrently.
> 
> The patch eliminates the race condition.

With this patch, the enum_sem field in struct hv_pcibus_device
is no longer needed.  The semaphore serializes execution in
hv_pci_devices_present_work(), and that serialization is now done
with the ordered workqueue.  Also, the last paragraph of the top level
comment for hv_pci_devices_present_work() should be updated to
reflect the new ordering assumptions.

Separately, an unrelated bug:  At the top of hv_eject_device_work(),
the first test may do a put_pcichild() and return.  This exit path also
needs to do put_hvpcibus() to balance the ref counts, or do a goto
the last two lines at the bottom of the function.

> 
> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Tested-by: Adrian Suhov <v-adsuho@xxxxxxxxxxxxx>
> Tested-by: Chris Valean <v-chvale@xxxxxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: Jack Morgenstein <jackm@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
> Cc: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> ---
>  drivers/pci/host/pci-hyperv.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/host/pci-hyperv.c b/drivers/pci/host/pci-hyperv.c
> index 1233300f41c6..57b1fb3ebdb9 100644
> --- a/drivers/pci/host/pci-hyperv.c
> +++ b/drivers/pci/host/pci-hyperv.c
> @@ -461,6 +461,8 @@ struct hv_pcibus_device {
>  	struct retarget_msi_interrupt retarget_msi_interrupt_params;
> 
>  	spinlock_t retarget_msi_interrupt_lock;
> +
> +	struct workqueue_struct *wq;
>  };
> 
>  /*
> @@ -1770,7 +1772,7 @@ static void hv_pci_devices_present(struct hv_pcibus_device *hbus,
>  	spin_unlock_irqrestore(&hbus->device_list_lock, flags);
> 
>  	get_hvpcibus(hbus);
> -	schedule_work(&dr_wrk->wrk);
> +	queue_work(hbus->wq, &dr_wrk->wrk);

This invocation of get_hvpcibus() and queue_work() could be made
conditional on whether the preceding list_add_tail() transitioned
the list from empty to non-empty.  If the list was already non-empty,
a previously queued invocation of hv_pci_devices_present_work()
will find the new entry and process it.   But this is an
optimization in a non-perf sensitive code path, so may not be
worth it.

>  }
> 
>  /**
> @@ -1848,7 +1850,7 @@ static void hv_pci_eject_device(struct hv_pci_dev *hpdev)
>  	get_pcichild(hpdev, hv_pcidev_ref_pnp);
>  	INIT_WORK(&hpdev->wrk, hv_eject_device_work);
>  	get_hvpcibus(hpdev->hbus);
> -	schedule_work(&hpdev->wrk);
> +	queue_work(hpdev->hbus->wq, &hpdev->wrk);
>  }
> 
>  /**
> @@ -2463,11 +2465,17 @@ static int hv_pci_probe(struct hv_device *hdev,
>  	spin_lock_init(&hbus->retarget_msi_interrupt_lock);
>  	sema_init(&hbus->enum_sem, 1);
>  	init_completion(&hbus->remove_event);
> +	hbus->wq = alloc_ordered_workqueue("hv_pci_%x", 0,
> +					   hbus->sysdata.domain);
> +	if (!hbus->wq) {
> +		ret = -ENOMEM;
> +		goto free_bus;
> +	}
> 
>  	ret = vmbus_open(hdev->channel, pci_ring_size, pci_ring_size, NULL, 0,
>  			 hv_pci_onchannelcallback, hbus);
>  	if (ret)
> -		goto free_bus;
> +		goto destroy_wq;
> 
>  	hv_set_drvdata(hdev, hbus);
> 
> @@ -2536,6 +2544,9 @@ static int hv_pci_probe(struct hv_device *hdev,
>  	hv_free_config_window(hbus);
>  close:
>  	vmbus_close(hdev->channel);
> +destroy_wq:
> +	drain_workqueue(hbus->wq);

The drain_workqueue() call isn't necessary.  destroy_workqueue() calls
drain_workqueue() and there better not be anything in the workqueue
anyway since all the ref counts are zero.

> +	destroy_workqueue(hbus->wq);
>  free_bus:
>  	free_page((unsigned long)hbus);
>  	return ret;
> @@ -2615,6 +2626,8 @@ static int hv_pci_remove(struct hv_device *hdev)
>  	irq_domain_free_fwnode(hbus->sysdata.fwnode);
>  	put_hvpcibus(hbus);
>  	wait_for_completion(&hbus->remove_event);
> +	drain_workqueue(hbus->wq);

Same here -- drain_workqueue() isn't needed.  The workqueue
must be empty anyway since the remove_event has completed
and the ref counts will all be zero.

> +	destroy_workqueue(hbus->wq);
>  	free_page((unsigned long)hbus);
>  	return 0;
>  }
> --
> 2.7.4