linux-kernel - Re: [PATCH 2/2] Drivers: hv: vmbus: Fix rescind handling in uio_hv

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Message-ID: <20240827182406.GA32019@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Tue, 27 Aug 2024 11:24:06 -0700
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>
Cc: Naman Jain <namjain@...ux.microsoft.com>,
	"K . Y . Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Stephen Hemminger <stephen@...workplumber.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] Drivers: hv: vmbus: Fix rescind handling in
 uio_hv_generic

On Mon, Aug 26, 2024 at 05:40:37AM +0000, Michael Kelley wrote:
> From: Naman Jain <namjain@...ux.microsoft.com> Sent: Sunday, August 25, 2024 10:32 PM
> > 
> > On 8/25/2024 8:27 AM, Michael Kelley wrote:
> > > From: Naman Jain <namjain@...ux.microsoft.com> Sent: Thursday, August 22, 2024 4:09 AM
> > >>
> > >> Rescind offer handling relies on rescind callbacks for some of the
> > >> resources cleanup, if they are registered. It does not unregister
> > >> vmbus device for the primary channel closure, when callback is
> > >> registered.
> > >> Add logic to unregister vmbus for the primary channel in rescind callback
> > >> to ensure channel removal and relid release, and to ensure rescind flag
> > >> is false when driver probe happens again.
> > >>
> > >> Fixes: ca3cda6fcf1e ("uio_hv_generic: add rescind support")
> > >> Signed-off-by: Naman Jain <namjain@...ux.microsoft.com>
> > >> ---
> > >>   drivers/hv/vmbus_drv.c       | 1 +
> > >>   drivers/uio/uio_hv_generic.c | 7 +++++++
> > >>   2 files changed, 8 insertions(+)
> > >>
> > >> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > >> index c857dc3975be..4bae382a3eb4 100644
> > >> --- a/drivers/hv/vmbus_drv.c
> > >> +++ b/drivers/hv/vmbus_drv.c
> > >> @@ -1952,6 +1952,7 @@ void vmbus_device_unregister(struct hv_device *device_obj)
> > >>   	 */
> > >>   	device_unregister(&device_obj->device);
> > >>   }
> > >> +EXPORT_SYMBOL_GPL(vmbus_device_unregister);
> > >>
> > >>   #ifdef CONFIG_ACPI
> > >>   /*
> > >> diff --git a/drivers/uio/uio_hv_generic.c b/drivers/uio/uio_hv_generic.c
> > >> index c99890c16d29..ea26c0b460d6 100644
> > >> --- a/drivers/uio/uio_hv_generic.c
> > >> +++ b/drivers/uio/uio_hv_generic.c
> > >> @@ -121,6 +121,13 @@ static void hv_uio_rescind(struct vmbus_channel *channel)
> > >>
> > >>   	/* Wake up reader */
> > >>   	uio_event_notify(&pdata->info);
> > >> +
> > >> +	/*
> > >> +	 * With rescind callback registered, rescind path will not unregister the device
> > >> +	 * when the primary channel is rescinded. Without it, next onoffer msg does not come.
> > >> +	 */
> > >> +	if (!channel->primary_channel)
> > >> +		vmbus_device_unregister(channel->device_obj);
> > >
> > > When the rescind callback is *not* set, vmbus_onoffer_rescind() makes the
> > > call to vmbus_device_unregister(). But it does so bracketed with get_device()/
> > > put_device(). Your code here does not do the bracketing. Is there a reason for
> > > the difference? Frankly, I'm not sure why vmbus_onoffer_rescind() does the
> > > bracketing, and I can't definitively say if it is really needed. So I guess I'm
> > > just asking if you know. :-)
> > >
> > > Michael
> > 
> > IMHO, we have already NULL checked channel->device_obj and other couple
> > of things to make sure we are safe to clean this up. At other places as
> > well, I don't see the use of put and get device. So I think its not
> > required. I am open to suggestions.
> > 
> > Regards,
> > Naman
> 
> OK. I'm good with what you've said, and don't have any further suggestions.
> Go with what your patch already has. :-)
> 
> Michael


Michael,

If we look at vmbus_onoffer_rescind function, hv_uio_rescind can only be called
if channel->device_obj is not NULL. By this if we conclude that hv_uio_rescind can
never be called for secondary channel I think we can simplify hv_uio_rescind
only for primary channel.

In the first patch of this series, instead of this:
+	struct hv_device *hv_dev = channel->primary_channel ?
+				   channel->primary_channel->device_obj : channel->device_obj;

We can only have

+	struct hv_device *hv_dev = channel->device_obj;


In second patch, instead of this:
+        if (!channel->primary_channel)
+                vmbus_device_unregister(channel->device_obj);

We can only have:
+                vmbus_device_unregister(channel->device_obj);
	

Possibly WARN for secondary channel is also not required as that will never happen ?


- Saurabh