On Sat, 05/23 19:11, Max Reitz wrote: > On 21.05.2015 08:43, Fam Zheng wrote: > >We don't want new requests from guest, so block the operation around the > >nested poll. > > > >It also avoids looping forever when iothread is submitting a lot of requests. > > > >Signed-off-by: Fam Zheng <f...@redhat.com> > >--- > > block/io.c | 22 ++++++++++++++++++++-- > > 1 file changed, 20 insertions(+), 2 deletions(-) > > Hm, I don't know about this. When I see someone calling > bdrv_drain()/bdrv_drain_all(), I'm expecting that every request has been > drained afterwards. This patch implies that this is not necessarily the > case, because apparently in some configurations the guest can still submit > I/O even while bdrv_drain() is running,
In dataplane, aio_poll in bdrv_drain_all will poll the ioeventfd, which could call the handlers of virtio queues. That's how guest I/O sneaks in. > but this means that even after this > patch, the same can happen if I/O is submitted after bdrv_op_unblock() and > before anything the caller of bdrv_drain() wants to do while the BDS is > still drained. So this looks to me more like the caller must ensure that the > BDS won't receive new requests, and do so before bdrv_drain() is called. Yes, callers of bdrv_drain*() should use a blocker like you reasoned. Other patches in this series looked at qmp_transaction, but there are more, which may still be wrong until they're fixed. This patch, however, fixes one of the potential issues of those callers: > >It also avoids looping forever when iothread is submitting a lot of > >requests. Fam
