RE: [PATCH v3 2/4] PCI: hv: Add the support of hibernation

[Michael Kelley <mikelley@microsoft.com>]

From: Dexuan Cui <decui@microsoft.com> Sent: Sunday, November 24, 2019 9:34 PM
> 
> Add suspend() and resume() functions so that Hyper-V virtual PCI devices are
> handled properly when the VM hibernates and resumes from hibernation.
> 
> Note that the suspend() function must make sure there are no pending work
> items before calling vmbus_close(), since it runs in a process context as a
> callback in dpm_suspend().  When it starts to run, the channel callback
> hv_pci_onchannelcallback(), which runs in a tasklet context, can be still running
> concurrently and scheduling new work items onto hbus->wq in
> hv_pci_devices_present() and hv_pci_eject_device(), and the work item
> handlers can access the vmbus channel, which can be being closed by
> hv_pci_suspend(), e.g. the work item handler pci_devices_present_work() ->
> new_pcichild_device() writes to the vmbus channel.
> 
> To eliminate the race, hv_pci_suspend() disables the channel callback
> tasklet, sets hbus->state to hv_pcibus_removing, and re-enables the tasklet.
> This way, when hv_pci_suspend() proceeds, it knows that no new work item
> can be scheduled, and then it flushes hbus->wq and safely closes the vmbus
> channel.
> 
> Signed-off-by: Dexuan Cui <decui@microsoft.com>

Reviewed-by: Michael Kelley <mikelley@microsoft.com>