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authorVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>2020-07-27 21:47:48 +0300
committerEric Blake <eblake@redhat.com>2020-07-28 09:54:43 -0500
commitdd1ec1a4afe190e030edfa052d95c9e6e065438c (patch)
tree2aa78e8482586126949db4568c7c9e2013d1f151
parentfa35591b9cb9a7fd0af2d8c2d8848abba30d3c69 (diff)
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block/nbd: allow drain during reconnect attempt
It should be safe to reenter qio_channel_yield() on io/channel read/write path, so it's safe to reduce in_flight and allow attaching new aio context. And no problem to allow drain itself: connection attempt is not a guest request. Moreover, if remote server is down, we can hang in negotiation, blocking drain section and provoking a dead lock. How to reproduce the dead lock: 1. Create nbd-fault-injector.conf with the following contents: [inject-error "mega1"] event=data io=readwrite when=before 2. In one terminal run nbd-fault-injector in a loop, like this: n=1; while true; do echo $n; ((n++)); ./nbd-fault-injector.py 127.0.0.1:10000 nbd-fault-injector.conf; done 3. In another terminal run qemu-io in a loop, like this: n=1; while true; do echo $n; ((n++)); ./qemu-io -c 'read 0 512' nbd://127.0.0.1:10000; done After some time, qemu-io will hang trying to drain, for example, like this: #3 aio_poll (ctx=0x55f006bdd890, blocking=true) at util/aio-posix.c:600 #4 bdrv_do_drained_begin (bs=0x55f006bea710, recursive=false, parent=0x0, ignore_bds_parents=false, poll=true) at block/io.c:427 #5 bdrv_drained_begin (bs=0x55f006bea710) at block/io.c:433 #6 blk_drain (blk=0x55f006befc80) at block/block-backend.c:1710 #7 blk_unref (blk=0x55f006befc80) at block/block-backend.c:498 #8 bdrv_open_inherit (filename=0x7fffba1563bc "nbd+tcp://127.0.0.1:10000", reference=0x0, options=0x55f006be86d0, flags=24578, parent=0x0, child_class=0x0, child_role=0, errp=0x7fffba154620) at block.c:3491 #9 bdrv_open (filename=0x7fffba1563bc "nbd+tcp://127.0.0.1:10000", reference=0x0, options=0x0, flags=16386, errp=0x7fffba154620) at block.c:3513 #10 blk_new_open (filename=0x7fffba1563bc "nbd+tcp://127.0.0.1:10000", reference=0x0, options=0x0, flags=16386, errp=0x7fffba154620) at block/block-backend.c:421 And connection_co stack like this: #0 qemu_coroutine_switch (from_=0x55f006bf2650, to_=0x7fe96e07d918, action=COROUTINE_YIELD) at util/coroutine-ucontext.c:302 #1 qemu_coroutine_yield () at util/qemu-coroutine.c:193 #2 qio_channel_yield (ioc=0x55f006bb3c20, condition=G_IO_IN) at io/channel.c:472 #3 qio_channel_readv_all_eof (ioc=0x55f006bb3c20, iov=0x7fe96d729bf0, niov=1, errp=0x7fe96d729eb0) at io/channel.c:110 #4 qio_channel_readv_all (ioc=0x55f006bb3c20, iov=0x7fe96d729bf0, niov=1, errp=0x7fe96d729eb0) at io/channel.c:143 #5 qio_channel_read_all (ioc=0x55f006bb3c20, buf=0x7fe96d729d28 "\300.\366\004\360U", buflen=8, errp=0x7fe96d729eb0) at io/channel.c:247 #6 nbd_read (ioc=0x55f006bb3c20, buffer=0x7fe96d729d28, size=8, desc=0x55f004f69644 "initial magic", errp=0x7fe96d729eb0) at /work/src/qemu/master/include/block/nbd.h:365 #7 nbd_read64 (ioc=0x55f006bb3c20, val=0x7fe96d729d28, desc=0x55f004f69644 "initial magic", errp=0x7fe96d729eb0) at /work/src/qemu/master/include/block/nbd.h:391 #8 nbd_start_negotiate (aio_context=0x55f006bdd890, ioc=0x55f006bb3c20, tlscreds=0x0, hostname=0x0, outioc=0x55f006bf19f8, structured_reply=true, zeroes=0x7fe96d729dca, errp=0x7fe96d729eb0) at nbd/client.c:904 #9 nbd_receive_negotiate (aio_context=0x55f006bdd890, ioc=0x55f006bb3c20, tlscreds=0x0, hostname=0x0, outioc=0x55f006bf19f8, info=0x55f006bf1a00, errp=0x7fe96d729eb0) at nbd/client.c:1032 #10 nbd_client_connect (bs=0x55f006bea710, errp=0x7fe96d729eb0) at block/nbd.c:1460 #11 nbd_reconnect_attempt (s=0x55f006bf19f0) at block/nbd.c:287 #12 nbd_co_reconnect_loop (s=0x55f006bf19f0) at block/nbd.c:309 #13 nbd_connection_entry (opaque=0x55f006bf19f0) at block/nbd.c:360 #14 coroutine_trampoline (i0=113190480, i1=22000) at util/coroutine-ucontext.c:173 Note, that the hang may be triggered by another bug, so the whole case is fixed only together with commit "block/nbd: on shutdown terminate connection attempt". Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> Message-Id: <20200727184751.15704-3-vsementsov@virtuozzo.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Eric Blake <eblake@redhat.com>
-rw-r--r--block/nbd.c14
1 files changed, 14 insertions, 0 deletions
diff --git a/block/nbd.c b/block/nbd.c
index 3558c17..ee9ab75 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -291,8 +291,22 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
goto out;
}
+ bdrv_dec_in_flight(s->bs);
+
ret = nbd_client_handshake(s->bs, sioc, &local_err);
+ if (s->drained) {
+ s->wait_drained_end = true;
+ while (s->drained) {
+ /*
+ * We may be entered once from nbd_client_attach_aio_context_bh
+ * and then from nbd_client_co_drain_end. So here is a loop.
+ */
+ qemu_coroutine_yield();
+ }
+ }
+ bdrv_inc_in_flight(s->bs);
+
out:
s->connect_status = ret;
error_free(s->connect_err);