| Message ID | 20240314165825.40261-2-kwolf@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | nbd: Fix server crash on reset with iothreads | expand |
14.03.2024 19:58, Kevin Wolf wrote: > When draining an NBD export, nbd_drained_begin() first sets > client->quiescing so that nbd_client_receive_next_request() won't start > any new request coroutines. Then nbd_drained_poll() tries to makes sure > that we wait for any existing request coroutines by checking that > client->nb_requests has become 0. > > However, there is a small window between creating a new request > coroutine and increasing client->nb_requests. If a coroutine is in this > state, it won't be waited for and drain returns too early. > > In the context of switching to a different AioContext, this means that > blk_aio_attached() will see client->recv_coroutine != NULL and fail its > assertion. > > Fix this by increasing client->nb_requests immediately when starting the > coroutine. Doing this after the checks if we should create a new > coroutine is okay because client->lock is held. > > Cc: qemu-stable@nongnu.org > Fixes: fd6afc501a019682d1b8468b562355a2887087bd > Signed-off-by: Kevin Wolf <kwolf@redhat.com> Kevin, Stefan, This change in master, which is Cc'ed stable, touches (refines) exactly the same areas as f816310d0c32c "nbd/server: only traverse NBDExport->clients from main loop thread", which is not (yet?) in stable, neither 7.2 nor 8.2. Also, 7075d235114b4 "nbd/server: introduce NBDClient->lock to protect fields" touches one of the places too. I can try to construct something out of the two, but I think it is better if either of you can do that, - if this seems a good thing to do anyway. This way it is definitely much saner than my possible attempts. Or we can just pick f816310d0c32c and 7075d235114b4 into stable too, - when I evaluated f816310d0c32c for stable before I thought it isn't needed there because AioContext lock isn't removed in 8.2 yet. And I haven't thought about 7075d235114b4 at all. All 3 applies cleanly and the result passes check-block, but it smells a bit too much for stable. What do you think? Thanks, /mjt > diff --git a/nbd/server.c b/nbd/server.c > index 941832f178..c3484cc1eb 100644 > --- a/nbd/server.c > +++ b/nbd/server.c > @@ -3007,8 +3007,8 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, > /* Owns a reference to the NBDClient passed as opaque. */ > static coroutine_fn void nbd_trip(void *opaque) > { > - NBDClient *client = opaque; > - NBDRequestData *req = NULL; > + NBDRequestData *req = opaque; > + NBDClient *client = req->client; > NBDRequest request = { 0 }; /* GCC thinks it can be used uninitialized */ > int ret; > Error *local_err = NULL; > @@ -3037,8 +3037,6 @@ static coroutine_fn void nbd_trip(void *opaque) > goto done; > } > > - req = nbd_request_get(client); > - > /* > * nbd_co_receive_request() returns -EAGAIN when nbd_drained_begin() has > * set client->quiescing but by the time we get back nbd_drained_end() may > @@ -3112,9 +3110,7 @@ static coroutine_fn void nbd_trip(void *opaque) > } > > done: > - if (req) { > - nbd_request_put(req); > - } > + nbd_request_put(req); > > qemu_mutex_unlock(&client->lock); > > @@ -3143,10 +3139,13 @@ disconnect: > */ > static void nbd_client_receive_next_request(NBDClient *client) > { > + NBDRequestData *req; > + > if (!client->recv_coroutine && client->nb_requests < MAX_NBD_REQUESTS && > !client->quiescing) { > nbd_client_get(client); > - client->recv_coroutine = qemu_coroutine_create(nbd_trip, client); > + req = nbd_request_get(client); > + client->recv_coroutine = qemu_coroutine_create(nbd_trip, req); > aio_co_schedule(client->exp->common.ctx, client->recv_coroutine); > } > }
diff --git a/nbd/server.c b/nbd/server.c index 941832f178..c3484cc1eb 100644 --- a/nbd/server.c +++ b/nbd/server.c @@ -3007,8 +3007,8 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, /* Owns a reference to the NBDClient passed as opaque. */ static coroutine_fn void nbd_trip(void *opaque) { - NBDClient *client = opaque; - NBDRequestData *req = NULL; + NBDRequestData *req = opaque; + NBDClient *client = req->client; NBDRequest request = { 0 }; /* GCC thinks it can be used uninitialized */ int ret; Error *local_err = NULL; @@ -3037,8 +3037,6 @@ static coroutine_fn void nbd_trip(void *opaque) goto done; } - req = nbd_request_get(client); - /* * nbd_co_receive_request() returns -EAGAIN when nbd_drained_begin() has * set client->quiescing but by the time we get back nbd_drained_end() may @@ -3112,9 +3110,7 @@ static coroutine_fn void nbd_trip(void *opaque) } done: - if (req) { - nbd_request_put(req); - } + nbd_request_put(req); qemu_mutex_unlock(&client->lock); @@ -3143,10 +3139,13 @@ disconnect: */ static void nbd_client_receive_next_request(NBDClient *client) { + NBDRequestData *req; + if (!client->recv_coroutine && client->nb_requests < MAX_NBD_REQUESTS && !client->quiescing) { nbd_client_get(client); - client->recv_coroutine = qemu_coroutine_create(nbd_trip, client); + req = nbd_request_get(client); + client->recv_coroutine = qemu_coroutine_create(nbd_trip, req); aio_co_schedule(client->exp->common.ctx, client->recv_coroutine); } }
When draining an NBD export, nbd_drained_begin() first sets client->quiescing so that nbd_client_receive_next_request() won't start any new request coroutines. Then nbd_drained_poll() tries to makes sure that we wait for any existing request coroutines by checking that client->nb_requests has become 0. However, there is a small window between creating a new request coroutine and increasing client->nb_requests. If a coroutine is in this state, it won't be waited for and drain returns too early. In the context of switching to a different AioContext, this means that blk_aio_attached() will see client->recv_coroutine != NULL and fail its assertion. Fix this by increasing client->nb_requests immediately when starting the coroutine. Doing this after the checks if we should create a new coroutine is okay because client->lock is held. Cc: qemu-stable@nongnu.org Fixes: fd6afc501a019682d1b8468b562355a2887087bd Signed-off-by: Kevin Wolf <kwolf@redhat.com> --- nbd/server.c | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-)