Am 12.07.2019 um 13:09 hat Max Reitz geschrieben: > On 12.07.19 13:01, Kevin Wolf wrote: > > Am 12.07.2019 um 12:47 hat Max Reitz geschrieben: > >> On 12.07.19 11:24, Kevin Wolf wrote: > >>> Am 11.07.2019 um 21:58 hat Max Reitz geschrieben: > >>>> When nbd_close() is called from a coroutine, the connection_co never > >>>> gets to run, and thus nbd_teardown_connection() hangs. > >>>> > >>>> This is because aio_co_enter() only puts the connection_co into the main > >>>> coroutine's wake-up queue, so this main coroutine needs to yield and > >>>> reschedule itself to let the connection_co run. > >>>> > >>>> Signed-off-by: Max Reitz <mre...@redhat.com> > >>>> --- > >>>> block/nbd.c | 12 +++++++++++- > >>>> 1 file changed, 11 insertions(+), 1 deletion(-) > >>>> > >>>> diff --git a/block/nbd.c b/block/nbd.c > >>>> index 81edabbf35..b83b6cd43e 100644 > >>>> --- a/block/nbd.c > >>>> +++ b/block/nbd.c > >>>> @@ -135,7 +135,17 @@ static void > >>>> nbd_teardown_connection(BlockDriverState *bs) > >>>> qio_channel_shutdown(s->ioc, > >>>> QIO_CHANNEL_SHUTDOWN_BOTH, > >>>> NULL); > >>>> - BDRV_POLL_WHILE(bs, s->connection_co); > >>>> + > >>>> + if (qemu_in_coroutine()) { > >>>> + /* Let our caller poll and just yield until connection_co is > >>>> done */ > >>>> + while (s->connection_co) { > >>>> + aio_co_schedule(qemu_get_current_aio_context(), > >>>> + qemu_coroutine_self()); > >>>> + qemu_coroutine_yield(); > >>>> + } > >>> > >>> Isn't this busy waiting? Why not let s->connection_co wake us up when > >>> it's about to terminate instead of immediately rescheduling ourselves? > >> > >> Yes, it is busy waiting, but I didn’t find that bad. The connection_co > >> will be invoked in basically every iteration, and once there is no > >> pending data, it will quit. > >> > >> The answer to “why not...” of course is because it’d be more complicated. > >> > >> But anyway. > >> > >> Adding a new function qemu_coroutine_run_after(target) that adds > >> qemu_coroutine_self() to the given @target coroutine’s wake-up queue and > >> then using that instead of scheduling works, too, yes. > >> > >> I don’t really like being responsible for coroutine code, though... > >> > >> (And maybe it’d be better to make it qemu_coroutine_yield_for(target), > >> which does the above and then yields?) > > > > Or just do something like this, which is arguably not only a fix for the > > busy wait, but also a code simplification: > > 1. Is that guaranteed to work? What if data sneaks in, the > connection_co handles that, and doesn’t wake up the teardown_co? Will > it be re-scheduled?
Then connection_co is buggy because we clearly requested that it terminate. It is possible that it does so only after handling another request, but this wouldn't be a problem. teardown_co would then just sleep for a few cycles more until connection_co is done and reaches the aio_co_wake() call. > 2. I precisely didn’t want to do this because we have this functionality > already in the form of Coroutine.co_queue_wakeup. Why duplicate it here? co_queue_wakeup contains coroutines to be run at the next yield point (or termination), which may be when connection_co is actually done, but it might also be earlier. My explicit aio_co_wake() at the end of connection_co is guaranteed to run only when connection_co is done. Kevin > > diff --git a/block/nbd.c b/block/nbd.c > > index b83b6cd43e..c061bd1bfc 100644 > > --- a/block/nbd.c > > +++ b/block/nbd.c > > @@ -61,6 +61,7 @@ typedef struct BDRVNBDState { > > CoMutex send_mutex; > > CoQueue free_sema; > > Coroutine *connection_co; > > + Coroutine *teardown_co; > > int in_flight; > > > > NBDClientRequest requests[MAX_NBD_REQUESTS]; > > @@ -137,12 +138,9 @@ static void nbd_teardown_connection(BlockDriverState > > *bs) > > NULL); > > > > if (qemu_in_coroutine()) { > > - /* Let our caller poll and just yield until connection_co is done > > */ > > - while (s->connection_co) { > > - aio_co_schedule(qemu_get_current_aio_context(), > > - qemu_coroutine_self()); > > - qemu_coroutine_yield(); > > - } > > + /* just yield until connection_co is done */ > > + s->teardown_co = qemu_coroutine_self(); > > + qemu_coroutine_yield(); > > } else { > > BDRV_POLL_WHILE(bs, s->connection_co); > > } > > @@ -217,6 +215,9 @@ static coroutine_fn void nbd_connection_entry(void > > *opaque) > > bdrv_dec_in_flight(s->bs); > > > > s->connection_co = NULL; > > + if (s->teardown_co) { > > + aio_co_wake(s->teardown_co); > > + } > > aio_wait_kick(); > > } > > > >
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