linux-kernel - RE: [PATCH v2 2/4] PCI: hv: Add the support of hibernation

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Message-ID: <PU1P153MB0169D0A99D5687FBDB02536BBF4E0@PU1P153MB0169.APCP153.PROD.OUTLOOK.COM>
Date:   Thu, 21 Nov 2019 00:50:17 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: RE: [PATCH v2 2/4] PCI: hv: Add the support of hibernation

> From: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Sent: Wednesday, November 20, 2019 9:20 AM
> 
> On Tue, Nov 19, 2019 at 11:16:56PM -0800, Dexuan Cui wrote:
> > Implement the suspend/resume callbacks.
> >
> > We must make sure there is no pending work items before we call
> > vmbus_close().
> 
> Where ? Why ? Imagine a developer reading this log to try to understand
> why you made this change, do you really think this commit log is
> informative in its current form ?
> 
> I am not asking a book but this is a significant feature please make
> an effort to explain it (I can update the log for you but please
> write one and I shall do it).
> 
> Lorenzo

Sorry for being sloppy on this patch's changelog! Can you please use the
below? I can also post v3 with the new changelog if that's better.

PCI: hv: Add the support of hibernation

hv_pci_suspend() 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.

Thanks,
-- Dexuan