Re: [PATCH 07/15] block-backend: enter aio coroutine only after drain

[Kevin Wolf <kwolf@redhat.com>]

Am 12.12.2022 um 13:59 hat Paolo Bonzini geschrieben:
> When called from within (another) coroutine, aio_co_enter will not
> enter a coroutine immediately; instead the new coroutine is scheduled
> to run after qemu_coroutine_yield().  This however might cause the
> currently-running coroutine to yield without having raised blk->in_flight.

I assume you're talking about the blk_aio_prwv() path here. However,
calling blk_inc_in_flight() is the very first thing it does (before even
calling bdrv_coroutine_enter -> aio_co_enter), so I don't understand how
it could happen that it yields before increasing the counter.

> If it was a ->drained_begin() callback who scheduled the coroutine,

Which one? The one that executes blk_aio_prwv()?

> bdrv_drained_begin() might exit without waiting for the I/O operation
> to finish.  Right now, this is masked by unnecessary polling done by
> bdrv_drained_begin() after the callbacks return, but it is wrong and
> a latent bug.
> 
> So, ensure that blk_inc_in_flight() and blk_wait_while_drained()
> are called before aio_co_enter().  To do so, pull the call to
> blk_wait_while_drained() out of the blk_co_do_* functions, which are
> called from the AIO coroutines, and place them separately in the public
> blk_co_* functions and in blk_aio_prwv.

You can't call blk_wait_while_drained() in blk_aio_prwv() because the
latter isn't a coroutine_fn.

> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/block-backend.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)

Kevin