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    SubjectRe: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure
    On Wed, Apr 22, 2020 at 10:01:34AM +0800, Ming Lei wrote:
    > On Wed, Apr 22, 2020 at 01:48:25AM +0000, Dexuan Cui wrote:
    > > > From: Ming Lei <ming.lei@redhat.com>
    > > > Sent: Tuesday, April 21, 2020 6:28 PM
    > > > To: Dexuan Cui <decui@microsoft.com>
    > > >
    > > > 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 there
    > > > > 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 = hv_get_drvdata(hv_dev);
    > > > > struct Scsi_Host *host = stor_device->host;
    > > > > struct hv_host_device *host_dev = shost_priv(host);
    > > > > + int ret;
    > > > > +
    > > > > + ret = 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 = hv_get_drvdata(hv_dev);
    > > > > + struct Scsi_Host *host = stor_device->host;
    > > > > int ret;
    > > > >
    > > > > ret = storvsc_connect_to_vsp(hv_dev, storvsc_ringbuffer_size,
    > > > > hv_dev_is_fc(hv_dev));
    > > > > + if (!ret)
    > > > > + ret = scsi_host_unblock(host, SDEV_RUNNING);
    > > > > +
    > > > > return ret;
    > > > > }
    > > >
    > > > scsi_host_block() is actually too heavy for just avoiding
    > > > scsi internal command, which can be done simply by one atomic
    > > > variable.
    > > >
    > > > 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.
    > > >
    > > > Also scsi_device_quiesce() is heavy too, still takes 2
    > > > synchronize_rcu() for one LUN.
    > > >
    > > > That is said SCSI suspend may take (3 * nr_luns) sysnchronize_rcu() in
    > > > case that the HBA's suspend handler needs scsi_host_block().
    > > >
    > > > 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.
    >
    > I guess it depends on RCU's implementation, so CC RCU guys.
    >
    > Hello Paul & Josh,
    >
    > Could you clarify that if sysnchronize_rcu becomes quickly during
    > system suspend?

    Once you have all but one CPU offlined, it becomes extremely fast, as
    in roughly a no-op (which is an idea of Josh's from back in the day).
    But if there is more than one CPU online, then synchronize_rcu() still
    takes on the order of several to several tens of jiffies.

    So, yes, in some portions of system suspend, synchronize_rcu() becomes
    very fast indeed.

    Thanx, Paul

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