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

[Tianyu Lan]

On 3/18/2020 1:35 AM, Wei Liu wrote:
> On Tue, Mar 17, 2020 at 06:25:20AM -0700, ltykernel@xxxxxxxxx wrote:
> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> >
> > Customer reported Hyper-V VM still responded network traffic
> > ack packets after kernel panic with kernel parameter "panic=0â.
> > 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@xxxxxxxxxxxxx>
> > ---
> >   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.