Re: [Qemu-devel] [RFC PATCH 09/16] qcow2: Move COW and L2 update into own coroutine

[Kevin Wolf <kwolf@redhat.com>]

Am 18.09.2012 16:24, schrieb Paolo Bonzini:
> Il 18/09/2012 13:40, Kevin Wolf ha scritto:
>> +            qemu_co_mutex_unlock(&s->lock);
>> +            qemu_co_rwlock_rdlock(&s->l2meta_flush);
> 
> Should this lock be taken in process_l2meta?  It's a bit easier to follow.

I'm pretty sure there was a reason, but it isn't obvious any more. I
guess I should have put a comment there... Maybe it doesn't exist any
more, or maybe it's not that obvious.

The difference would be that while waiting for the lock, the original
write request could complete instead of waiting as well, and that the
lock is potentially taken only in a BH instead of immediately.

What happens if bdrv_aio_flush() and bdrv_aio_writev() are both in
flight? If the flush runs its stop_l2meta() after the write request has
signalled completion, but before the COW coroutine has started, it gets
the lock even though a COW must still be processed. I believe we could
then return a successful flush when the metadata isn't really on disk yet.

So if you agree, I think we need to leave it where it is.

Kevin