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

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Message-ID: <a447911b-ef12-46de-ba01-13105e34b8fe@linux.microsoft.com>
Date: Mon, 26 Aug 2024 11:01:39 +0530
From: Naman Jain <namjain@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.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>
Cc: "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Saurabh Sengar <ssengar@...ux.microsoft.com>
Subject: Re: [PATCH 2/2] Drivers: hv: vmbus: Fix rescind handling in
 uio_hv_generic



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
> 
>>   }
>>
>>   /* ysfs API to allow mmap of the ring buffers
>> --
>> 2.34.1
>>

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