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17:11:41 +0100 Date: Sat, 14 Feb 2026 17:11:29 +0100 From: Lukas Straub To: Fabiano Rosas Cc: qemu-devel@nongnu.org, Peter Xu , Zhang Chen , Hailiang Zhang , Li Zhijian , "Dr. David Alan Gilbert" , stefanha@redhat.com Subject: Re: COLO concurrency issues Message-ID: <20260214171129.29b35bd9@penguin> In-Reply-To: <87ms1cn8n6.fsf@suse.de> References: <87ms1cn8n6.fsf@suse.de> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="Sig_/f4iOBFQbbqd4DuEk=f_bz=+"; protocol="application/pgp-signature"; micalg=pgp-sha512 X-Provags-ID: V03:K1:KuPoXqx4FSSNYsJPmfYIFSACtcPh1+0zmZuVb3rFyFJklsWIMGn vCjssoM7KNTDcH1FB9rE5FZsqSV84PKL6SzhGhr+SLkUEvPq+ck0ywKHx4m4O9AvloAlt8a WT6Sagi4a083GslpGCqKYIDnGjTWPpFqr0Qf6utBGGf33oEiY1E8uphkD3tqOm8e1dHM4YD mgK7HVz3zXGCsFUN3qI/Q== UI-OutboundReport: notjunk:1;M01:P0:48FwdLGHDaQ=;t8v5UQpbMKyCwCJe2UBVMrQC5Jx WmSXrGVL2exuyMszK8B3nDlw+7Y+wubHM8qdiITvIMv+hRy8ZsHr8ToiDEfT5P12vu4NQDUvY DgUnT8uqFZKeWEVpp+WgwAH86NXVtZfKncfxxVlVD1PoZ3I2FVeRNwPBkdyyWjdmMHaHwV/e9 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qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --Sig_/f4iOBFQbbqd4DuEk=f_bz=+ Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Fri, 13 Feb 2026 09:13:49 -0300 Fabiano Rosas wrote: > Hi, I've been following the qemu-colo.rst steps to test COLO and > encountered a couple of issues. Unfortunately, I don't have cycles to > investigate further. Happens with QEMU master (also tested some versions > back until the COLO fix 0b5bf4ea76). >=20 > 1) Deadlock at fdmon_io_uring_wait: >=20 > (steps from qemu-colo.rst) > - Secondary Failover > - Secondary resume replication > - Start the new Secondary > - Sync > - Wait until disk is synced, then: >=20 > {"execute": "stop"} > {"execute": "block-job-cancel", "arguments":{ "device": "resync" } } >=20 > The above results in the old secondary hanging indefinitely at: >=20 > do { > ret =3D io_uring_submit_and_wait(&ctx->fdmon_io_uring, wait_nr); > } while (ret =3D=3D -EINTR); I tried and I can not reproduce this at all with my patchset on top of 0b91040d23dc8820724a60c811223b777f3bc6b7 How often does this happen for you? The only colo-specific culprint at this step could be the replication block driver. It feels like this should be reproducible without colo, can you try the following: 1. start secondary ./build/qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=3Don -m 512 -sm= p 1 -qmp stdio -device piix3-usb-uhci -device usb-tablet -name secondary= -netdev user,id=3Dhn0 -device rtl8139,id=3De0,netdev=3Dhn0 -drive if= =3Dide,id=3Dparent0,file.filename=3D$imagefolder/primary.qcow2,driver=3Dqco= w2 -incoming tcp:0.0.0.0:9998 2. qmp commands on secondary {"execute":"qmp_capabilities"} {"execute": "migrate-set-capabilities", "arguments": {"capabilities": [ {"c= apability": "x-colo", "state": true } ] } } {"execute": "nbd-server-start", "arguments": {"addr": {"type": "inet", "dat= a": {"host": "0.0.0.0", "port": "9999"} } } } {"execute": "nbd-server-add", "arguments": {"device": "parent0", "writable"= : true } } 3. start primary ./build/qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=3Don -m 512 -sm= p 1 -qmp stdio -device piix3-usb-uhci -device usb-tablet -name secondary= -netdev user,id=3Dhn0 -device rtl8139,id=3De0,netdev=3Dhn0 -drive if= =3Dide,id=3Dparent0,file.filename=3D$imagefolder/primary.qcow2,driver=3Dqco= w2 4. qmp commands on primary {"execute":"qmp_capabilities"} {"execute": "drive-mirror", "arguments":{ "device": "parent0", "job-id": "r= esync", "target": "nbd://127.0.0.1:9999/parent0", "mode": "existing", "form= at": "raw", "sync": "full"} } 5. wait for resync, then on primary {"execute": "stop"} {"execute": "block-job-cancel", "arguments":{ "device": "resync" } } >=20 > (gdb) bt = = =20 > #0 syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38 > #1 0x00007f5519e0204e in ??? () at //usr/lib64/liburing.so.2 > #2 0x00007f5519e01b00 in ??? () at //usr/lib64/liburing.so.2 > #3 0x0000563c2dc06cc9 in fdmon_io_uring_wait (ctx=3D0x563c30411b00, read= y_list=3D0x7ffd0bad8f58, timeout=3D575708467831) at ../util/fdmon-io_uring.= c:416 > #4 0x0000563c2dc00976 in aio_poll (ctx=3D0x563c30411b00, blocking=3Dtrue= ) at ../util/aio-posix.c:699 > #5 0x0000563c2daa01c6 in bdrv_drain_all_begin () at ../block/io.c:529 > #6 0x0000563c2daa03d8 in bdrv_drain_all () at ../block/io.c:574 > #7 0x0000563c2d764aae in do_vm_stop (state=3DRUN_STATE_PAUSED, send_stop= =3Dtrue) at ../system/cpus.c:312 > #8 0x0000563c2d765964 in vm_stop (state=3DRUN_STATE_PAUSED) at ../system= /cpus.c:754 > #9 0x0000563c2d7f3378 in qmp_stop (errp=3D0x7ffd0bad9080) at ../monitor/= qmp-cmds.c:62 > #10 0x0000563c2dba7a72 in qmp_marshal_stop (args=3D0x563c306ac070, ret=3D= 0x7f5518dffda8, errp=3D0x7f5518dffda0) at qapi/qapi-commands-misc.c:197 > #11 0x0000563c2dbf1316 in do_qmp_dispatch_bh (opaque=3D0x7f5518dffe40) at= ../qapi/qmp-dispatch.c:128 > #12 0x0000563c2dc1de48 in aio_bh_call (bh=3D0x563c3040fef0) at ../util/as= ync.c:173 > #13 0x0000563c2dc1df64 in aio_bh_poll (ctx=3D0x563c3040c070) at ../util/a= sync.c:220 > #14 0x0000563c2dbffff0 in aio_dispatch (ctx=3D0x563c3040c070) at ../util/= aio-posix.c:389 > #15 0x0000563c2dc1e3cd in aio_ctx_dispatch (source=3D0x563c3040c070, call= back=3D0x0, user_data=3D0x0) at ../util/async.c:365 > #16 0x00007f551b114f4c in g_main_dispatch (context=3D0x563c304120f0) at .= ./glib/gmain.c:3476 > #17 g_main_context_dispatch_unlocked (context=3Dcontext@entry=3D0x563c304= 120f0) at ../glib/gmain.c:4284 > #18 0x00007f551b1170c9 in g_main_context_dispatch (context=3D0x563c304120= f0) at ../glib/gmain.c:4272 > #19 0x0000563c2dc1fa0b in glib_pollfds_poll () at ../util/main-loop.c:290 > #20 0x0000563c2dc1fa85 in os_host_main_loop_wait (timeout=3D0) at ../util= /main-loop.c:313 > #21 0x0000563c2dc1fb8a in main_loop_wait (nonblocking=3D0) at ../util/mai= n-loop.c:592 > #22 0x0000563c2d78eb60 in qemu_main_loop () at ../system/runstate.c:903 > #23 0x0000563c2db412fc in qemu_default_main (opaque=3D0x0) at ../system/m= ain.c:50 > #24 0x0000563c2db413ab in main (argc=3D40, argv=3D0x7ffd0bad94d8) at ../s= ystem/main.c:93 >=20 > --- >=20 > 2) Race at colo_process_checkpoint >=20 > The following pattern seems to be inherently racy, whether the switch > statement sees the state as COMPLETED or not varies: >=20 > colo_process_checkpoint() > { > ... > out: > ... > /* > * There are only two reasons we can get here, some error happened > * or the user triggered failover. > */ > --> switch (failover_get_state()) { =20 > case FAILOVER_STATUS_COMPLETED: > qapi_event_send_colo_exit(COLO_MODE_PRIMARY, > COLO_EXIT_REASON_REQUEST); > break; > default: > qapi_event_send_colo_exit(COLO_MODE_PRIMARY, > COLO_EXIT_REASON_ERROR); > } >=20 > /* Hope this not to be too long to wait here */ > --> qemu_event_wait(&s->colo_exit_event); =20 > ... > } >=20 > This results in what seems like a spurious: >=20 > {"timestamp": {"seconds": 1770984655, "microseconds": 216464}, "event": > "COLO_EXIT", "data": {"mode": "primary", "reason": "error"}} >=20 > I'm not sure if the intention is to just ignore it, but it seems moving > the qemu_event_wait before checking the state would eliminate the race. >=20 The issue is, if the connection breaks without triggering failover we want to be notified of that. I want to rework this anyway, I think I can simplify failover and remove colo-failover.c. --Sig_/f4iOBFQbbqd4DuEk=f_bz=+ Content-Type: application/pgp-signature Content-Description: OpenPGP digital signature -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEg/qxWKDZuPtyYo+kNasLKJxdslgFAmmQnrEACgkQNasLKJxd slie3g//XeeiR3ypz94xNYp1B0L/FC28Xy2rcOsXoLSWK5xCASKyo4O+3nHOYK5r YyAtlXYoroBaWzXOihTsyLnAAzgjFkMv0JQpKLgd55HMPnn5iklJJRCibn40MxI3 lHQR1Ba7TlWi8nmpMRTE91HmEG1QFcpwn5OeUySO25qDBhxDjnjo3Q9Zyv2GlDoz iKl26Tvy9eLRM4LwHVW5eyPTVK1pW6oi8Qz5fu9TJLfD9Q8Jd1W/vXQNkmXqYglN UJB3gjpTwvM47Ou4v1wEh7Z3ZtUHBAY8ve/zxgRti5/eumYpYJ5+LXApHxCGLiNf air9vsczhcO5xXlKuTSTO7nrWqmgL/3DSH7wlmVDnAPg9MF5iey5PfXeROGMtxWF +ZsgzhE6z2B5ljQ55ocZzKfn2O+xfOxIh35YRh5bJNHjAjIfmIG37z6Tdos+TQlf KwxoYgEVjZdnK1T65aQYXkwhWC63l9qAhALPpxhHWx53UQrEOIY7UsoJBHG8lSgp szVi0uuBAGrl7LqKAfjv5LSf15FvTH2RqRHCV/2ZbTfHnoYxAhEnNxPdWs8s27qr pGqk3It0EDAuD5dLEIaq8ddtwmxu5MNnuvoxkCdKUGMaovfQFJslRh+CMBIOIcCh ZJTiaogd/R3crxs+Vqfn1SEvxnotDkK1Xii6o3dwE1nWxsZ55oE= =Quu/ -----END PGP SIGNATURE----- --Sig_/f4iOBFQbbqd4DuEk=f_bz=+--