[Qemu-devel] Re: Endless loop in qcow2_alloc_cluster_offset

[Jan Kiszka]

Jan Kiszka wrote:
> Kevin Wolf wrote:
>> Hi Jan,
>>
>> Am 19.11.2009 13:19, schrieb Jan Kiszka:
>>> (gdb) print ((BDRVQcowState *)bs->opaque)->cluster_allocs.lh_first 
>>> $5 = (struct QCowL2Meta *) 0xcb3568
>>> (gdb) print *((BDRVQcowState *)bs->opaque)->cluster_allocs.lh_first 
>>> $6 = {offset = 7417176064, n_start = 0, nb_available = 16, nb_clusters = 0, 
>>> depends_on = 0xcb3568, dependent_requests = {lh_first = 0x0}, 
>>> next_in_flight = {le_next = 0xcb3568, le_prev = 0xc4ebd8}}
>>>
>>> So next == first.
>> Oops. Doesn't sound quite right...
>>
>>> Is something fiddling with cluster_allocs concurrently, e.g. some signal
>>> handler? Or what could cause this list corruption? Would it be enough to
>>> move to QLIST_FOREACH_SAFE?
>> Are there any specific signals you're thinking of? Related to block code
> 
> No, was just blind guessing.
> 
>> I can only think of SIGUSR2 and this one shouldn't call any block driver
>> functions directly. You're using aio=threads, I assume? (It's the default)
> 
> Yes, all on defaults.
> 
>> QLIST_FOREACH_SAFE shouldn't make a difference in this place as the loop
>> doesn't insert or remove any elements. If the list is corrupted now, I
>> think it would be corrupted with QLIST_FOREACH_SAFE as well - at best,
>> the endless loop would occur one call later.
>>
>> The only way I see to get such a loop in a list is to re-insert an
>> element that already is part of the list. The only insert is at
>> qcow2-cluster.c:777. Remains the question how we came there twice
>> without run_dependent_requests() removing the L2Meta from our list first
>> - because this is definitely wrong...
>>
>> Presumably, it's not reproducible?
> 
> Likely not. What I did was nothing special, and I did not noticed such a
> crash in the last months.

And now it happened again (qemu-kvm head, during kernel installation
from network onto local qcow2-disk). Any clever idea how to proceed with
this?

I could try to run the step in a loop, hopefully retriggering it once in
a (likely longer) while. But then we need some good instrumentation first.

Jan

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