Re: [PATCH] Drivers: hv: vmbus: handle various crash scenarios

[Vitaly Kuznetsov <vkuznets@redhat.com>]

KY Srinivasan <kys@microsoft.com> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
>> Sent: Friday, March 18, 2016 5:33 AM
>> To: devel@linuxdriverproject.org
>> Cc: linux-kernel@vger.kernel.org; KY Srinivasan <kys@microsoft.com>;
>> Haiyang Zhang <haiyangz@microsoft.com>; Alex Ng (LIS)
>> <alexng@microsoft.com>; Radim Krcmar <rkrcmar@redhat.com>; Cathy
>> Avery <cavery@redhat.com>
>> Subject: [PATCH] Drivers: hv: vmbus: handle various crash scenarios
>> 
>> Kdump keeps biting. Turns out CHANNELMSG_UNLOAD_RESPONSE is always
>> delivered to CPU0 regardless of what CPU we're sending
>> CHANNELMSG_UNLOAD
>> from. vmbus_wait_for_unload() doesn't account for the fact that in case
>> we're crashing on some other CPU and CPU0 is still alive and operational
>> CHANNELMSG_UNLOAD_RESPONSE will be delivered there completing
>> vmbus_connection.unload_event, our wait on the current CPU will never
>> end.
>
> What was the host you were testing on?
>

I was testing on both 2012R2 and 2016TP4. The bug is easily reproducible
by forcing crash on a secondary CPU, e.g.:

# cat crash.sh
#! /bin/sh
echo c > /proc/sysrq-trigger

# taskset -c 1 ./crash.sh

>> 
>> Do the following:
>> 1) Check for completion_done() in the loop. In case interrupt handler is
>>    still alive we'll get the confirmation we need.
>> 
>> 2) Always read CPU0's message page as CHANNELMSG_UNLOAD_RESPONSE
>> will be
>>    delivered there. We can race with still-alive interrupt handler doing
>>    the same but we don't care as we're checking completion_done() now.
>> 
>> 3) Cleanup message pages on all CPUs. This is required (at least for the
>>    current CPU as we're clearing CPU0 messages now but we may want to
>> bring
>>    up additional CPUs on crash) as new messages won't be delivered till we
>>    consume what's pending. On boot we'll place message pages somewhere
>> else
>>    and we won't be able to read stale messages.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  drivers/hv/channel_mgmt.c | 30 +++++++++++++++++++++++++-----
>>  1 file changed, 25 insertions(+), 5 deletions(-)
>> 
>> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
>> index b10e8f74..5f37057 100644
>> --- a/drivers/hv/channel_mgmt.c
>> +++ b/drivers/hv/channel_mgmt.c
>> @@ -512,14 +512,26 @@ static void init_vp_index(struct vmbus_channel
>> *channel, const uuid_le *type_gui
>> 
>>  static void vmbus_wait_for_unload(void)
>>  {
>> -	int cpu = smp_processor_id();
>> -	void *page_addr = hv_context.synic_message_page[cpu];
>> +	int cpu;
>> +	void *page_addr = hv_context.synic_message_page[0];
>>  	struct hv_message *msg = (struct hv_message *)page_addr +
>>  				  VMBUS_MESSAGE_SINT;
>>  	struct vmbus_channel_message_header *hdr;
>>  	bool unloaded = false;
>> 
>> -	while (1) {
>> +	/*
>> +	 * CHANNELMSG_UNLOAD_RESPONSE is always delivered to CPU0.
>> When we're
>> +	 * crashing on a different CPU let's hope that IRQ handler on CPU0 is
>> +	 * still functional and vmbus_unload_response() will complete
>> +	 * vmbus_connection.unload_event. If not, the last thing we can do
>> is
>> +	 * read message page for CPU0 regardless of what CPU we're on.
>> +	 */
>> +	while (!unloaded) {
>> +		if (completion_done(&vmbus_connection.unload_event)) {
>> +			unloaded = true;
>> +			break;
>> +		}
>> +
>>  		if (READ_ONCE(msg->header.message_type) ==
>> HVMSG_NONE) {
>>  			mdelay(10);
>>  			continue;
>> @@ -530,9 +542,17 @@ static void vmbus_wait_for_unload(void)
>>  			unloaded = true;
>> 
>>  		vmbus_signal_eom(msg);
>> +	}
>> 
>> -		if (unloaded)
>> -			break;
>> +	/*
>> +	 * We're crashing and already got the UNLOAD_RESPONSE, cleanup
>> all
>> +	 * maybe-pending messages on all CPUs to be able to receive new
>> +	 * messages after we reconnect.
>> +	 */
>> +	for_each_online_cpu(cpu) {
>> +		page_addr = hv_context.synic_message_page[cpu];
>> +		msg = (struct hv_message *)page_addr +
>> VMBUS_MESSAGE_SINT;
>> +		msg->header.message_type = HVMSG_NONE;
>>  	}
>>  }
>> 
>> --
>> 2.5.0

-- 
  Vitaly