Re: [PATCH 0/4] x86/Hyper-V: Unload vmbus channel in hv panic callback

[Tianyu Lan <ltykernel@gmail.com>]

On 3/18/2020 1:35 AM, Wei Liu wrote:
> On Tue, Mar 17, 2020 at 06:25:20AM -0700, ltykernel@gmail.com wrote:
>> From: Tianyu Lan <Tianyu.Lan@microsoft.com>
>>
>> Customer reported Hyper-V VM still responded network traffic
>> ack packets after kernel panic with kernel parameter "panic=0”.
>> This becauses vmbus driver interrupt handler still works
>> on the panic cpu after kernel panic. Panic cpu falls into
>> infinite loop of panic() with interrupt enabled at that point.
>> Vmbus driver can still handle network traffic.
>>
>> This confuses remote service that the panic system is still
>> alive when it gets ack packets. Unload vmbus channel in hv panic
>> callback and fix it.
>>
>> vmbus_initiate_unload() maybe double called during panic process
>> (e.g, hyperv_panic_event() and hv_crash_handler()). So check
>> and set connection state in vmbus_initiate_unload() to resolve
>> reenter issue.
>>
>> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
>> ---
>>   drivers/hv/channel_mgmt.c |  5 +++++
>>   drivers/hv/vmbus_drv.c    | 17 +++++++++--------
>>   2 files changed, 14 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
>> index 0370364169c4..893493f2b420 100644
>> --- a/drivers/hv/channel_mgmt.c
>> +++ b/drivers/hv/channel_mgmt.c
>> @@ -839,6 +839,9 @@ void vmbus_initiate_unload(bool crash)
>>   {
>>   	struct vmbus_channel_message_header hdr;
>>   
>> +	if (vmbus_connection.conn_state == DISCONNECTED)
>> +		return;
>> +
>>   	/* Pre-Win2012R2 hosts don't support reconnect */
>>   	if (vmbus_proto_version < VERSION_WIN8_1)
>>   		return;
>> @@ -857,6 +860,8 @@ void vmbus_initiate_unload(bool crash)
>>   		wait_for_completion(&vmbus_connection.unload_event);
>>   	else
>>   		vmbus_wait_for_unload();
>> +
>> +	vmbus_connection.conn_state = DISCONNECTED;
> 
> This is only set at the end of the function.  I don't see how this solve
> the re-entrant issue with the check at the beginning. Do I miss anything
> here?
> 

For this issue, vmbus_initiate_unload() maybe called on the panic vcpu
twice and so just split check and set conn_state.

> Maybe this function should check and set the state to
> DISCONNECTING/DISCONNECTED at the beginning of this function?
> 
Yes, Vitaly also gave suggestion to use "xchg" to check and set
conn_state. Will update in the next version.