Am 30.03.2022 um 11:58 hat Emanuele Giuseppe Esposito geschrieben: > > > Am 30/03/2022 um 11:52 schrieb Vladimir Sementsov-Ogievskiy: > > 30.03.2022 12:09, Emanuele Giuseppe Esposito wrote: > >>> > >>> 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. > >> > > > > Hm, I don't see, where is global queue here? Or > > bdrv_wait_while_drained() is global and has no bs arguement? > > > > > > From what I understand, blk_wait_while_drained has a queue internally. > Yes, the queue would be global, and all coroutines that want to perform > a read will have to wait until the modification is ended. > > Whether to wake the queue up with a drain or a write lock is also > another point worth discussion maybe.
I think I intended it to be per-BDS (just like blk_wait_while_drained() uses a per-BB queue). Obviously, you can always replace a finer grained lock with a coarse one without losing correctness. I'm not sure if that simplifies things enough to justify using a single global lock (which would be a step backwards even from AioContext locks). On the other hand, graph modifications are rare, so maybe it's not too bad. Kevin
