On 24/01/2017 13:04, Fam Zheng wrote: > > All in all I think we should skip external handlers regardless of > aio_disable_external(), or even skip try_poll_mode, in nested aio_poll()'s. > The > reasons are 1) many nested aio_poll()'s don't have bdrv_drained_begin, so this > check is not sufficient [...] bdrv_flush() > spin longer than necessary, if not forever, when the guest keeps submitting > more > requests with ioeventfd.
I'm not sure I understand why this is related. aio_poll() only tries poll mode once, so bdrv_flush would only spin until the fsync is complete. Nested aio_polls don't have bdrv_drained_begin because draining matters over the whole section where you need atomicity (e.g. taking a live snapshot). It doesn't matter for single I/O operation. Paolo
