Re: [PATCH v2] block/stream: Drain subtree around graph change

[Hanna Reitz <hreitz@redhat.com>]

On 25.03.22 17:37, Vladimir Sementsov-Ogievskiy wrote:
> 24.03.2022 17:09, Hanna Reitz wrote:
>> When the stream block job cuts out the nodes between top and base in
>> stream_prepare(), it does not drain the subtree manually; it fetches the
>> base node, and tries to insert it as the top node's backing node with
>> bdrv_set_backing_hd().  bdrv_set_backing_hd() however will drain, and so
>> the actual base node might change (because the base node is actually not
>> part of the stream job) before the old base node passed to
>> bdrv_set_backing_hd() is installed.
>>
>> This has two implications:
>>
>> First, the stream job does not keep a strong reference to the base node.
>> Therefore, if it is deleted in bdrv_set_backing_hd()'s drain (e.g.
>> because some other block job is drained to finish), we will get a
>> use-after-free.  We should keep a strong reference to that node.
>>
>> Second, even with such a strong reference, the problem remains that the
>> base node might change before bdrv_set_backing_hd() actually runs and as
>> a result the wrong base node is installed.
>
> Hmm.
>
> So, we don't really need a strong reference, as if it helps to avoid 
> some use-after-free, it means that we'll finish up with wrong block 
> graph..

Sure.  But I found it better style to strongly reference a node while 
it’s used.  I’d rather have an outdated block graph (as in: A node that 
was supposed to disappear would still be in use) than a use-after-free.

> Graph modifying operations must be somehow isolated from each other.
>
>>
>> Both effects can be seen in 030's TestParallelOps.test_overlapping_5()
>> case, which has five nodes, and simultaneously streams from the middle
>> node to the top node, and commits the middle node down to the base node.
>> As it is, this will sometimes crash, namely when we encounter the
>> above-described use-after-free.
>>
>> Taking a strong reference to the base node, we no longer get a crash,
>> but the resuling block graph is less than ideal: The expected result is
>> obviously that all middle nodes are cut out and the base node is the
>> immediate backing child of the top node.  However, if stream_prepare()
>> takes a strong reference to its base node (the middle node), and then
>> the commit job finishes in bdrv_set_backing_hd(), supposedly dropping
>> that middle node, the stream job will just reinstall it again.
>>
>> Therefore, we need to keep the whole subtree drained in
>> stream_prepare(), so that the graph modification it performs is
>> effectively atomic, i.e. that the base node it fetches is still the base
>> node when bdrv_set_backing_hd() sets it as the top node's backing node.
>
> Emanuele has similar idea of isolating graph changes from each other 
> by subtree-drain.
>
> If I understand correctly the idea is that we'll drain all other block 
> jobs, so the wouldn't do their block-graph modification during drained 
> section. So, we can safely modify the graph.
>
> I don't like this idea:
>
> 1. drained section = stop IO. But we don't need to stop IO in the 
> whole subtree to do a needed block-graph modification.

If you mean to say that draining just the single node should be 
sufficient, I’ll be happy to change it.

Not sure which node, though, because I’d think it would be `base`, but 
to safely fetch it I’d need to drain it, which seems to bite itself in 
the tail.  That’s why I went for a subtree drain from `above_base`.

> 2. Drained section is not a lock, several clients may drain same set 
> of nodes.. So we exploit the fact that concurrent clients will be 
> paused by drained section and don't proceed to graph-modification 
> code.. But are we sure that block-jobs are (and will be?) the only 
> concurrent block-graph modifying clients? Can qmp commands interleave 
> somehow?

They can under very specific circumstances and that’s a bug.  See 
https://lists.nongnu.org/archive/html/qemu-block/2022-03/msg00582.html .

> Can some jobs from other subtree start a block-graph modification that 
> touches our subtree?

That would be wrong.  A block job shouldn’t change nodes it doesn’t own; 
stream doesn’t own the base, but it also doesn’t change it, it only 
needs to have the top node point to it.

> If go this way, that would be more safe to drain the whole block-graph 
> on any block-graph modification..
>
> I think we'd better have a separate global mechanism for isolating 
> graph modifications. Something like a global co-mutex or queue, where 
> clients waits for their turn in block graph modifications.
>
> Here is my old proposal on that topic: 
> https://patchew.org/QEMU/20201120161622.1537-1-vsementsov@virtuozzo.com/

That would only solve the very specific issue in 030, right?  The stream 
job isn’t protected from any graph modifications but those coming from 
mirror.  Might be a solution going forward (I didn’t look closer at it 
at the time, given I saw you had a discussion with Kevin), if we lock 
every graph change operation (though a global lock honestly doesn’t 
sound strictly better than draining subsections of the graph, both have 
their drawbacks), but that doesn’t look like it’d be something for 7.1.