Re: [PATCH v2 00/11] Refactor bdrv_try_set_aio_context using transactions

[Vladimir Sementsov-Ogievskiy]

On 8/5/22 16:22, Emanuele Giuseppe Esposito wrote:
> Am 27/07/2022 um 18:13 schrieb Vladimir Sementsov-Ogievskiy:
> > On 7/25/22 15:21, Emanuele Giuseppe Esposito wrote:
> > > The aim of this series is to reorganize bdrv_try_set_aio_context
> > > and drop BDS ->set_aio_context and ->can_set_aio_ctx callbacks in
> > > favour of a new one, ->change_aio_ctx.
> > >
> > > More informations in patch 3 (which is also RFC, due to the doubts
> > > I have with AioContext locks).
> > >
> > > Patch 1 just add assertions in the code, 2 extends the transactions
> > > API to be able to add also transactions in the tail
> > > of the list.
> > > Patch 3 is the core of this series, and introduces the new callback.
> > > It is marked as RFC and the reason is explained in the commit message.
> > > Patches 4-5-6 implement ->change_aio_ctx in the various block, blockjob
> > > and block-backend BDSes.
> > > Patch 7 substitutes ->change_aio_ctx with the old callbacks, and
> > > patch 8 takes care of deleting the old callbacks and unused code.
> > >
> > > This series is based on "job: replace AioContext lock with job_mutex",
> > > but just because it uses job_set_aio_context() introduced there.
> > >
> > > Suggested-by: Paolo Bonzini<pbonzini@redhat.com>
> > > Based-on:<20220706201533.289775-1-eesposit@redhat.com>
>
> So, I read your email before going on PTO and at that point I got what
> your concerns were, but now after re-reading it I don't understand
> anymore what you mean :)
>
> > What I dislike here is that you refactor aio-context-change to use
> > transactions, but you use it "internally" for aio-context-change.
> > aio-context-change doesn't become a native part of block-graph
> > modifiction transaction system after the series.
> >
> > For example, bdrv_attach_child_common(..., tran) still calls
> > bdrv_try_change_aio_context() function which doesn't take @tran
> > argument. And we have to call bdrv_try_change_aio_context() again in
> > bdrv_attach_child_common_abort() with opposite arguments by hand. We
> > create in _try_ separate Transaction object which is unrelated to the
> > original block-graph-change transaction.
>
> This can be fixed: patch 4 "bdrv_child_try_change_aio_context: add
> transaction parameter" supports taking transaction as a parameter.
> bdrv_attach_child_common could simply call
> bdrv_try_change_aio_context_tran (ok we need to deal with locking, but
> it could work).
>
> I think the main concern here is that during the prepare phase this
> serie doesn't change any aiocontext, so until we don't commit the rest
> of the code cannot assume that the aiocontext has been changed.
>
> But isn't it what happens also for permissions? Permission functions
> like bdrv_drv_set_perm perform bdrv_check_perm() in .prepare(), and then
> bdrv_set_perm() in commit.

Not exactly.

Partly that's just old bad naming. Ideally, driver handlers should be 
refactored to have one
.bdrv_set_perm(, ... tran, errp) handler. Or at least renamed to .prepare and 
.commit instead of misleading .check and .set.

Secondly what is important, is that corresponding .set actions are not visible 
to other block-graph modifying actions (like taking locks on fd. other actions, 
like attach/detach children don't care of it)/ (Or, at least, they _shoud not 
be_ visible :) To be honest, I don't have real expertise, of how much these 
.set actions are visible or not by other block-graph modifying actions, but I 
believe that we are OK in common scenarios).

What is really visible at generic block layer? Visible is change of .perm / 
.shared_perm fields of BdrvChild. And they are set in .prepare, look at 
bdrv_child_set_perm().

So, after calling bdrv_child_set_perm() other actions of .prepare stage will 
see _updated_ permissions. And if transaction fails, we rollback .perm and 
.shared_perm fields in bdrv_child_set_perm_abort()


> Another important question is that if we actually want to put everything
> in a single transaction, because .prepare functions like the one
> proposed here perform drains, so the logic following prepare and
> preceding commit must take into account that everything is drained. Also
> prepare itself has to take into account that maybe other .prepare took
> locks or drained themselves...

Yes, untie the knot of drained sections during aio context change is a 
challenge.. And that's (at least on of) the reason why aio-context change is 
still not a native part of block graph modifying transactions.

Could there be some simple solution?

Like

1. drain the whole subgraph of all nodes connected with nodes that we are going 
to touch

2. do block graph modifying transaction (including aio context change) knowing 
that everything we need is drained. (so we don't have to care about drain 
during aio context change)

3. undrain the subgraph

In other words, is that really necessary to lock/unlock different contexts 
during the aio-context-change procedure? Or we can move to a lot larger and 
simpler drained section?

> > With good refactoring we should get rid of these _try_ functions, and
> > have just bdrv_change_aio_context(..., tran) that can be natively used
> > with external tran object, where other block-graph change actions
> > participate. This way we'll not have to call reverse
> > aio_context_change() in .abort, it will be done automatically.
> >
> > Moreover, your  *aio_context_change* functions that do have tran
> > parameter cannot be simply used in the block-graph change transaction,
> > as you don't follow the common paradigm, that in .prepare we do all
> > visible changes. That's why you have to still use _try_*() version that
> > creates seaparate transaction object and completes it: after that the
> > action is actually done and other graph-modifying actions can be done on
> > top.
> >
> > So, IMHO, we shouldn't go this way, as that adds transaction actions
> > that actually can't be used in common block-graph-modifying transaction
> > but only context of bdrv_try_change_aio_context() internal transaction.
> >
> > I agree that aio-context change should finally be rewritten to take a
> > native place in block-graph transactions, but that should be a complete
> > solution, introducing native bdrv_change_aio_context(..., tran)
> > transaction action that is directly used in the block-graph transaction,
> > do visible effect in .prepare and don't create extra Transaction objects.


--
Best regards,
Vladimir