Re: [PATCH] block/file-posix: don't use functions calling AIO_WAIT_WHILE in worker threads

[Kevin Wolf <kwolf@redhat.com>]

Am 09.02.2023 um 16:45 hat Emanuele Giuseppe Esposito geschrieben:
> When calling bdrv_getlength() in handle_aiocb_write_zeroes(), the
> function creates a new coroutine and then waits that it finishes using
> AIO_WAIT_WHILE.
> The problem is that this function could also run in a worker thread,
> that has a different AioContext from main loop and iothreads, therefore
> in AIO_WAIT_WHILE we will have in_aio_context_home_thread(ctx) == false
> and therefore
> assert(qemu_get_current_aio_context() == qemu_get_aio_context());
> in the else branch will fail, crashing QEMU.
> 
> Aside from that, bdrv_getlength() is wrong also conceptually, because
> it reads the BDS graph from another thread and is not protected by
> any lock.
> 
> Replace it with raw_co_getlength, that doesn't create a coroutine and
> doesn't read the BDS graph.
> 
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>  block/file-posix.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index d3073a7caa..9a99111f45 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1738,7 +1738,7 @@ static int handle_aiocb_write_zeroes(void *opaque)
>  #ifdef CONFIG_FALLOCATE
>      /* Last resort: we are trying to extend the file with zeroed data. This
>       * can be done via fallocate(fd, 0) */
> -    len = bdrv_getlength(aiocb->bs);
> +    len = raw_co_getlength(aiocb->bs);
>      if (s->has_fallocate && len >= 0 && aiocb->aio_offset >= len) {
>          int ret = do_fallocate(s->fd, 0, aiocb->aio_offset, aiocb->aio_nbytes);
>          if (ret == 0 || ret != -ENOTSUP) {

Obviously this relies on the fact that raw_co_getlength() doesn't
actually depend on running in coroutine context. Could be done in a
separate patch, but I think we should rename it back to raw_getlength()
and remove the coroutine_fn annotation again. Seems commit c86422c5549
was a little too eager.

Kevin