Re: [PATCH v2 30/36] block: bdrv_reopen_multiple: refresh permissions on updated graph

[Kevin Wolf]

Am 08.02.2021 um 12:21 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > Come to think of it, the AioContext handling is probably wrong already
> > before your series. reopen_commit for one node could move the whole tree
> > to a different context and then the later nodes would all be processed
> > while holding the wrong lock.
> 
> Probably proper way is to acquire all involved aio contexts as I do in
> 29 and update aio-context updating functions to work in such
> conditions(all aio contexts are already acquired by caller).

Whoops, what I gave was kind of a non-answer...

So essentially the reason for the locking rules of changing the
AioContext is that they drain the node first and drain imposes the
locking rule that the AioContext for the node to be drained must be
locked, and all other AioContexts must be unlocked.

The reason why drain imposes the rule is that we run AIO_WAIT_WHILE() in
one thread and we may need the event loops in other threads to make
progress until the while condition can eventually become false. If other
threads can't make progress because their lock is taken, we'll see
deadlocks sooner or later.

Kevin