Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

>
> Ah seems I understand what you mean.
>
> One of my arguments is that "drain" - is not a lock against other
> clients who want to modify the graph. Because, drained section allows
> nested drained sections.
>
> And you try to solve it, by draining more things, this way, we'll drain
> also the job, which is a possible client, who may want to modify the
> graph in parallel.
>
> So, in other words, when we want to modify the graph, we drain the whole
> connectivity component of the graph. And we think that we are safe from
> other graph modifications because all related jobs are drained.
> Interesting, is that possible that some not drained job from another
> connectivity component will want to connect some node from our drained
> component?

You mean another job or whathever calling bdrv_find_node() on a random
graph? Yes that is not protected. But can this happen?

That's the question. What are the invariants here? Can anything happen?

>
> I just still feel that draining is a wrong mechanism to avoid
> interaction with other clients who want to modify the graph, because:
>
> 1. we stop the whole IO on all subgraph which is not necessary
> 2. draining is not a mutex, it allows nesting and it's ok when two
> different clients drain same nodes. Draining is just a requirement to do
> no IO at these nodes.
>
> And in your way, it seems that to be absolutely safe we'll need to drain
> everything..
>
> In my feeling it's better to keep draining what it is now: requirement
> to have no IO requests. And to isolate graph modifications from each
> other make a new synchronization mechanism, something like a global
> queue, where clients who want to get an access to graph modifications
> wait for their turn.

This is a matter of definitions. Subtree drains can theoretically work,
I managed to answer to my own doubts in the last email I sent.

Yes, there is still some completely random case like the one I wrote
above, but I think it is more a matter of what we want to use and what
meaning we want to give to drains.

Global queue is what Kevin proposes, I will try to implement it.

> 
> 
> As I understand:
> 
> You want to make drained section to be a kind of lock, so that if we
> take this lock, we can modify the graph and we are sure that no other
> client will modify it in parallel.

Yes

> 
> But drained sections can be nested. So to solve the problem you try to
> drain more nodes: include subtree for example, or may be we need to
> drain the whole graph connectivity component, or (to be more safe) the
> whole block layer (to be sure that during drained section in one
> connectivity component some not-drained block-job from another
> connectivity component will not try to attach some node from our drained
> connectivity component)..
> 
> I still feel that draining is wrong tool for isolating graph modifying
> operations from each other:
> 
> 1. Drained sections can be nested, and natively that's not a kind of
> lock. That's just a requirement to have no IO requests. There may be
> several clients that want this condition on same set of nodes.
> 
> 2. Blocking IO on the whole connected subgraph or even on the whole
> block layer graph is not necessary, so that's an extra blocking.
> 
> 
> Could we instead do the following:
> 
> 1. Keep draining as is - a mechanism to stop IO on some nodes
> 
> 2. To isolate graph-modifying operations implement another mechanism:
> something like a global queue, where clients wait until they gen an
> access to modify block layer.
> 
> 
> This way, any graph modifying process would look like this:
> 
> 1. drained_begin(only where necessary, not the whole subgraph in general)
> 
> 2. wait in the global queue
> 
> 3. Ok, now we can do all the modifications
> 
> 4. Kick the global queue, so that next client will get an access
> 
> 5. drained_end()
> 
> 

Please give a look at what Kevin (described by me) proposed. I think
it's the same as you are suggesting. I am pasting it below.
I will try to implement this and see if it is doable or not.

I think the advantage of drains is that it isn't so complicated and
doesn't add any complication to the existing code.
But we'll see how it goes with this global queue.

> His idea is to replicate what blk_wait_while_drained() currently does
> but on a larger scale. It is something in between this subtree_drains
> logic and a rwlock.
> 
> Basically if I understood correctly, we could implement
> bdrv_wait_while_drained(), and put in all places where we would put a
> read lock: all the reads to ->parents and ->children.
> This function detects if the bdrv is under drain, and if so it will stop
> and wait that the drain finishes (ie the graph modification).
> On the other side, each write would just need to drain probably both
> nodes (simple drain), to signal that we are modifying the graph. Once
> bdrv_drained_begin() finishes, we are sure all coroutines are stopped.
> Once bdrv_drained_end() finishes, we automatically let all coroutine
> restart, and continue where they left off.
> 
> Seems a good compromise between drains and rwlock. What do you think?
> 
> I am not sure how painful it will be to implement though.

Emanuele