Re: [PATCH v1 1/1] vfio-ccw: Don't free channel programs for unrelated interrupts

Eric Farman
2020-01-29T16:48:40Z

On 1/29/20 7:00 AM, Cornelia Huck wrote:
> On Tue, 28 Jan 2020 23:13:30 -0500
> Eric Farman <farman@linux.ibm.com> wrote:
>
>> On 1/28/20 9:42 AM, Eric Farman wrote:
>>>
>>>
>>> On 1/28/20 4:58 AM, Cornelia Huck wrote:
>>>> On Mon, 27 Jan 2020 16:28:18 -0500
>>
>> ...snip...
>>
>>>>
>>>> cp_init checking cp->initialized would probably be good to catch
>>>> errors, in any case. (Maybe put a trace there, just to see if it fires?)
>>>
>>> I did this last night, and got frustrated. The unfortunate thing was
>>> that once it fires, we end up flooding our trace buffers with errors as
>>> the guest continually retries. So I need to either make a smarter trace
>>> that is rate limited or just crash my host once this condition occurs.
>>> Will try to do that between meetings today.
>>>
>>
>> I reverted the subject patch, and simply triggered
>> BUG_ON(cp->initialized) in cp_init(). It sprung VERY quickly (all
>> traces are for the same device):
>>
>> 366.399682 03 ...sch_io_todo state=4 o.cpa=03017810
>> i.w0=00c04007 i.cpa=03017818 i.w2=0c000000
>> 366.399832 03 ...sch_io_todo state=3 o.cpa=7f53dd30 UNSOLICITED
>> i.w0=00c00011 i.cpa=03017818 i.w2=85000000
>> 366.400086 03 ...sch_io_todo state=2 o.cpa=03017930
>> i.w0=00c04007 i.cpa=03017938 i.w2=0c000000
>> 366.400313 03 ...sch_io_todo state=3 o.cpa=03017930
>> i.w0=00001001 i.cpa=03017938 i.w2=00000000
>>
>> Ah, of course... Unsolicited interrupts DO reset private->state back to
>> idle, but leave cp->initialized and any channel_program struct remains
>> allocated. So there's one problem (a memory leak), and an easy one to
>> rectify.
>
> For a moment, I suspected a deferred condition code here, but it seems
> to be a pure unsolicited interrupt.
>
> But that got me thinking: If we get an unsolicited interrupt while
> building the cp, it means that the guest is currently executing ssch.
> We need to get the unsolicited interrupt to the guest, while not
> executing the ssch. So maybe we need to do the following:
>
> - deliver the unsolicited interrupt to the guest
> - make sure we don't execute the ssch, but relay a cc 1 for it back to
> the guest
> - clean up the cp
>
> Maybe not avoiding issuing the ssch is what gets us in that pickle? We
> either leak memory or free too much, it seems.

It's possible... I'll try hacking at that for a bit.

>
>>
>> After more than a few silly rabbit holes, I had this trace:
>>
>> 429.928480 07 ...sch_io_todo state=4 init=1 o.cpa=7fed8e10
>> i.w0=00001001 i.cpa=7fed8e18 i.w2=00000000
>> 429.929132 07 ...sch_io_todo state=4 init=1 o.cpa=0305aed0
>> i.w0=00c04007 i.cpa=0305aed8 i.w2=0c000000
>> 429.929538 07 ...sch_io_todo state=4 init=1 o.cpa=0305af30
>> i.w0=00c04007 i.cpa=0305af38 i.w2=0c000000
>> 467.339389 07 ...chp_event mask=0x80 event=1
>> 467.339865 03 ...sch_io_todo state=3 init=0 o.cpa=01814548
>> i.w0=00c02001 i.cpa=0305af38 i.w2=00000000
>>
>> So my trace is at the beginning of vfio_ccw_sch_io_todo(), but the
>> BUG_ON() is at the end of that function where private->state is
>> (possibly) updated. Looking at the contents of the vfio_ccw_private
>> struct in the dump, the failing device is currently state=4 init=1
>> instead of 3/0 as in the above trace. So an I/O was being built in
>> parallel here, and there's no serializing action within the stacked
>> vfio_ccw_sch_io_todo() call to ensure they don't stomp on one another.
>> The io_mutex handles the region changes, and the subchannel lock handles
>> the start/halt/clear subchannel instructions, but nothing on the
>> interrupt side, nor contention between them. Sigh.
>
> I feel we've been here a few times already, and never seem to come up
> with a complete solution :(
>
> There had been some changes by Pierre regarding locking the fsm; maybe
> that's what's needed here?

Hrm... I'd forgotten all about those. I found them on
patchwork.kernel.org; will see what they encompass.

>
>>
>> My brain hurts. I re-applied this patch (with some validation that the
>> cpa is valid) to my current franken-code, and will let it run overnight.
>> I think it's going to be racing other CPUs and I'll find a dead system
>> by morning, but who knows. Maybe not. :)
>>
>
> I can relate to the brain hurting part :)
>

:)

My system crashed after about six hours, but not because of the BUG_ON()
traps I placed. Rather, dma-kmalloc-8 got clobbered again with what
looks like x100 bytes of data from one of the other CCWs. Of course, I
didn't trace the CCW/IDA data this time, so I don't know when the memory
in question was allocated/released/used. But, there are 35 deferred
cc=1 interrupts in the trace though, so I'll give some some thought to
the ideas above while re-running with the full traces in place.

Thanks!