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

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