On Tue, 02/23 11:43, Paolo Bonzini wrote: > > > On 23/02/2016 06:57, Fam Zheng wrote: > >>>> + qed_cancel_need_check_timer(s); > >>>> + qed_need_check_timer_cb(s); > >>>> + } > >>> > >>> What if an allocating write is queued (the else branch case)? Its > >>> completion > >>> will be in bdrv_drain and it could arm the need_check_timer which is > >>> wrong. > >>> > >>> We need to drain the allocating_write_reqs queue before checking the > >>> timer. > >> > >> You're right, but how? That's what bdrv_drain(bs) does, it's a > >> chicken-and-egg problem. > > > > Maybe use an aio_poll loop before the if? > > That would not change the fact that you're reimplementing bdrv_drain > inside bdrv_qed_drain. >
But it fulfills the contract of .bdrv_drain. This is the easy way, the hard way would be iterating through the allocating_write_reqs list and process reqs one by one synchronously, which still involves aio_poll indirectly. > Perhaps for now it's simplest to just remove the QED .bdrv_drain > callback, if you think this patch is not a good stopgap measure to avoid > the segmentation faults. OK, I'm fine with this as a stopgap measure. > > Once the bdrv_drain rework is in, we can move the callback _after_ I/O > is drained on bs and before it is drained on bs->file->bs. Sounds good. Fam