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

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Message-ID:
 <SN6PR02MB4157E8227056DCA579A9E3EBD4952@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Wed, 28 Aug 2024 14:53:56 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Saurabh Singh Sengar <ssengar@...ux.microsoft.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

From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com> Sent: Tuesday, August 27, 2024 11:24 AM
> 
> 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;
> 

Agreed. That was the intent of my previous comments on the first patch.

> 
> 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);
> 

Agreed.

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

Agreed -- the WARN is optional. I'm OK with leaving it out. But please
leave a comment in both places that the function is only called for
the primary channel, so it's not necessary to do any checking of the
primary_channel field. Future readers of the code will thank you. :-)

Michael


> 
> - Saurabh