{"id":2226560,"url":"http://patchwork.ozlabs.org/api/1.1/covers/2226560/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/cover/20260422161202.34150-1-viking4@gmail.com/","project":{"id":14,"url":"http://patchwork.ozlabs.org/api/1.1/projects/14/?format=json","name":"QEMU Development","link_name":"qemu-devel","list_id":"qemu-devel.nongnu.org","list_email":"qemu-devel@nongnu.org","web_url":"","scm_url":"","webscm_url":""},"msgid":"<20260422161202.34150-1-viking4@gmail.com>","date":"2026-04-22T16:12:01","name":"[0/1] migration/multifd: fix channel count TOCTOU race on cancel and retry","submitter":{"id":92831,"url":"http://patchwork.ozlabs.org/api/1.1/people/92831/?format=json","name":"Trieu Huynh","email":"vikingtc4@gmail.com"},"mbox":"http://patchwork.ozlabs.org/project/qemu-devel/cover/20260422161202.34150-1-viking4@gmail.com/mbox/","series":[{"id":501039,"url":"http://patchwork.ozlabs.org/api/1.1/series/501039/?format=json","web_url":"http://patchwork.ozlabs.org/project/qemu-devel/list/?series=501039","date":"2026-04-22T16:12:01","name":"migration/multifd: fix channel count TOCTOU race on cancel and retry","version":1,"mbox":"http://patchwork.ozlabs.org/series/501039/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/covers/2226560/comments/","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=cPkWmns+;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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helo=mail-pf1-x434.google.com","X-Spam_score_int":"-17","X-Spam_score":"-1.8","X-Spam_bar":"-","X-Spam_report":"(-1.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1,\n DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1,\n FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001,\n RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001,\n SPF_PASS=-0.001 autolearn=ham autolearn_force=no","X-Spam_action":"no action","X-BeenThere":"qemu-devel@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"},"content":"From: Trieu Huynh \n\nQEMU aborts on src during multifd migration when multifd-channels is\nchanged after migrate_cancel:\n\n~ # qemu-system-x86_64: ../util/yank.c:107: yank_unregister_instance:\nAssertion `QLIST_EMPTY(&entry->yankfns)' failed.\nAborted (core dumped)\n\nBelow is the backtrace:\n#7 yank_unregister_instance (instance=0x7fffffffd870) at ../util/yank.c:107\n#8 migration_cleanup (s=0x555557649d10) at ../migration/migration.c:1338\n#9 migration_cleanup_bh (opaque=0x555557649d10) at ../migration/migration.c:1345\n#10 migration_bh_dispatch_bh (opaque=0x7ffe500051d0) at ../migration/migration.c:350\n#11 aio_bh_call (bh=0x7ffe5006d670) at ../util/async.c:173\n#12 aio_bh_poll (ctx=0x555557649210) at ../util/async.c:220\n#13 aio_dispatch (ctx=0x555557649210) at ../util/aio-posix.c:390\n#14 aio_ctx_dispatch (source=0x555557649210) at ../util/async.c:365\n...\n\nSteps to reproduce:\n\n* 1. Start VM on src\n./qemu-system-x86_64 \\\n -\n\n* 2. Start dest with -incoming defer\n./qemu-system-x86_64 \\\n -incoming defer \\\n -\n\n* 3. Enable multifd with 4 channels on both src and dest\n{ \"execute\": \"qmp_capabilities\" }\n{ \"execute\": \"migrate-set-capabilities\",\n \"arguments\": {\"capabilities\": [{\"capability\": \"multifd\",\n \"state\": true}]} }\n{ \"execute\": \"migrate-set-parameters\",\n \"arguments\": {\"multifd-channels\": 4} }\n{ \"execute\": \"migrate-incoming\",\n \"arguments\": {\"uri\": \"tcp:0:4444\"} }\n\n* 4. Trigger migration\n** dest:\n{ \"execute\": \"migrate-incoming\", \"arguments\": {\"uri\": \"tcp:0:4444\"} }\n** src:\n{ \"execute\": \"migrate\", \"arguments\": {\"uri\": \"tcp:127.0.0.1:4444\"} }\n\n* 5. Cancel migration on src while status is active\n{ \"execute\": \"migrate_cancel\" }\n\n* 6. Immediately change multifd-channels to 2 on src\n{ \"execute\": \"migrate-set-parameters\",\n \"arguments\": {\"multifd-channels\": 2} }\n\nQEMU aborts as shown above\n\nTrieu Huynh (1):\n migration/multifd: fix channel count TOCTOU race on cancel and retry\n\n migration/multifd.c | 13 ++++++++-----\n 1 file changed, 8 insertions(+), 5 deletions(-)"}