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From:   Dexuan Cui <decui@microsoft.com>
To:     Ming Lei <ming.lei@redhat.com>
CC:     "jejb@linux.ibm.com" <jejb@linux.ibm.com>,
        "martin.petersen@oracle.com" <martin.petersen@oracle.com>,
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        "bvanassche@acm.org" <bvanassche@acm.org>,
        "hare@suse.de" <hare@suse.de>,
        Michael Kelley <mikelley@microsoft.com>,
        Long Li <longli@microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>
Subject: RE: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure
Thread-Topic: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure
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Date:   Wed, 22 Apr 2020 01:48:25 +0000
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> From: Ming Lei <ming.lei@redhat.com>
> Sent: Tuesday, April 21, 2020 6:28 PM
> To: Dexuan Cui <decui@microsoft.com>
>=20
> On Tue, Apr 21, 2020 at 05:17:24PM -0700, Dexuan Cui wrote:
> > During hibernation, the sdevs are suspended automatically in
> > drivers/scsi/scsi_pm.c before storvsc_suspend(), so after
> > storvsc_suspend(), there is no disk I/O from the file systems, but ther=
e
> > can still be disk I/O from the kernel space, e.g. disk_check_events() -=
>
> > sr_block_check_events() -> cdrom_check_events() can still submit I/O
> > to the storvsc driver, which causes a paic of NULL pointer dereference,
> > since storvsc has closed the vmbus channel in storvsc_suspend(): refer
> > to the below links for more info:
> >
> > Fix the panic by blocking/unblocking all the I/O queues properly.
> >
> > Note: this patch depends on another patch "scsi: core: Allow the state
> > change from SDEV_QUIESCE to SDEV_BLOCK" (refer to the second link
> above).
> >
> > Fixes: 56fb10585934 ("scsi: storvsc: Add the support of hibernation")
> > Signed-off-by: Dexuan Cui <decui@microsoft.com>
> > ---
> >  drivers/scsi/storvsc_drv.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> > index fb41636519ee..fd51d2f03778 100644
> > --- a/drivers/scsi/storvsc_drv.c
> > +++ b/drivers/scsi/storvsc_drv.c
> > @@ -1948,6 +1948,11 @@ static int storvsc_suspend(struct hv_device
> *hv_dev)
> >  	struct storvsc_device *stor_device =3D hv_get_drvdata(hv_dev);
> >  	struct Scsi_Host *host =3D stor_device->host;
> >  	struct hv_host_device *host_dev =3D shost_priv(host);
> > +	int ret;
> > +
> > +	ret =3D scsi_host_block(host);
> > +	if (ret)
> > +		return ret;
> >
> >  	storvsc_wait_to_drain(stor_device);
> >
> > @@ -1968,10 +1973,15 @@ static int storvsc_suspend(struct hv_device
> *hv_dev)
> >
> >  static int storvsc_resume(struct hv_device *hv_dev)
> >  {
> > +	struct storvsc_device *stor_device =3D hv_get_drvdata(hv_dev);
> > +	struct Scsi_Host *host =3D stor_device->host;
> >  	int ret;
> >
> >  	ret =3D storvsc_connect_to_vsp(hv_dev, storvsc_ringbuffer_size,
> >  				     hv_dev_is_fc(hv_dev));
> > +	if (!ret)
> > +		ret =3D scsi_host_unblock(host, SDEV_RUNNING);
> > +
> >  	return ret;
> >  }
>=20
> scsi_host_block() is actually too heavy for just avoiding
> scsi internal command, which can be done simply by one atomic
> variable.
>=20
> Not mention scsi_host_block() is implemented too clumsy because
> nr_luns * synchronize_rcu() are required in scsi_host_block(),
> which should have been optimized to just one.
>=20
> Also scsi_device_quiesce() is heavy too, still takes 2
> synchronize_rcu() for one LUN.
>=20
> That is said SCSI suspend may take (3 * nr_luns) sysnchronize_rcu() in
> case that the HBA's suspend handler needs scsi_host_block().
>=20
> Thanks,
> Ming

When we're in storvsc_suspend(), all the userspace processes have been
frozen and all the file systems have been flushed, and there should not
be too much I/O from the kernel space, so IMO scsi_host_block() should be
pretty fast here.=20

Of cource, any performance improvement to the scsi_host_block() API is
still great. :-)

Thanks,
-- Dexuan