Re: [Qemu-devel] [PATCH 02/18] block: BDS deletion during bdrv_drain_recurse

[Max Reitz <mreitz@redhat.com>]

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On 2017-09-18 18:13, Max Reitz wrote:
> On 2017-09-18 05:44, Fam Zheng wrote:
>> On Wed, 09/13 20:18, Max Reitz wrote:
>>> Drainined a BDS child may lead to both the original BDS and/or its other
>>> children being deleted (e.g. if the original BDS represents a block
>>> job).  We should prepare for this in both bdrv_drain_recurse() and
>>> bdrv_drained_begin() by monitoring whether the BDS we are about to drain
>>> still exists at all.
>>
>> Can the deletion happen when IOThread calls
>> bdrv_drain_recurse/bdrv_drained_begin?
> 
> I don't think so, because (1) my issue was draining a block job and that
> can only be completed in the main loop, and (2) I would like to think
> it's always impossible, considering that bdrv_unref() may only be called
> with the BQL.
> 
>>                                         If not, is it enough to do
>>
>>     ...
>>     if (in_main_loop) {
>>         bdrv_ref(bs);
>>     }
>>     ...
>>     if (in_main_loop) {
>>         bdrv_unref(bs);
>>     }
>>
>> to protect the main loop case? So the BdrvDeletedStatus state is not needed.
> 
> We already have that in bdrv_drained_recurse(), don't we?
> 
> The issue here is, though, that QLIST_FOREACH_SAFE() stores the next
> child pointer to @tmp.  However, once the current child @child is
> drained, @tmp may no longer be valid -- it may have been detached from
> @bs, and it may even have been deleted.
> 
> We could work around the latter by increasing the next child's reference
> somehow (but BdrvChild doesn't really have a refcount, and in order to
> do so, we would probably have to emulate being a parent or
> something...), but then you'd still have the issue of @tmp being
> detached from the children list we're trying to iterate over.  So
> tmp->next is no longer valid.
> 
> Anyway, so the latter is the reason why I decided to introduce the bs_list.
> 
> But maybe that actually saves us from having to fiddle with BdrvChild...
>  Since it's just a list of BDSs now, it may be enough to simply
> bdrv_ref() all of the BDSs in that list before draining any of them.  So
>  we'd keep creating the bs_list and then we'd move the existing
> bdrv_ref() from the drain loop into the loop filling bs_list.
> 
> And adding a bdrv_ref()/bdrv_unref() pair to bdrv_drained_begin() should
> hopefully work there, too.

It turns out it isn't so simple after all... because bdrv_close()
invokes bdrv_drained_begin(). So we may end up with an endless recursion
here.

One way to fix this would be to skip the bdrv_drained_begin() in
bdrv_close() if this would result in such a recursion...  But any
solution that comes quickly to my mind would require another BDS field,
too -- just checking the quiesce_counter is probably not enough because
this might just indicate concurrent drainage that stops before
bdrv_close() wants it to stop.

So maybe BdrvDeletedStatus is the simplest solution after all...?

Max


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