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: 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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