[Qemu-devel] Re: Endless loop in qcow2_alloc_cluster_offset

[Jan Kiszka]

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.

Jan

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