From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 2CE41D3B7E7 for ; Mon, 8 Dec 2025 13:51:55 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vSbe3-0003uS-Sb; Mon, 08 Dec 2025 08:51:19 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vSbe2-0003qv-2f for qemu-devel@nongnu.org; Mon, 08 Dec 2025 08:51:18 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vSbe0-0000ZQ-FZ for qemu-devel@nongnu.org; Mon, 08 Dec 2025 08:51:17 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1765201875; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=7snRi4k6OL50UuXEsAz1w2MO1BL+GPtsicaFpNiI3xo=; b=IVQlpTaBJImzYNdAcoIDSLH6MRM2sTsUdsgBKBVPy3LpdbzFauXzt+zmQ/GLlWNjmqvm2e Mk8fr6xK/fEjZs8G7ToIgGAvEcMgZBzhyUrNX8BIYDsrcOCBoO6NYjtMCJivQwFaXebNEi YeL2DN614jS9s3fRGDVvw36T0njWShU= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-180-U1s6szEFNcivNt7eV_JTbw-1; Mon, 08 Dec 2025 08:51:12 -0500 X-MC-Unique: U1s6szEFNcivNt7eV_JTbw-1 X-Mimecast-MFC-AGG-ID: U1s6szEFNcivNt7eV_JTbw_1765201870 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 7062B18002ED; Mon, 8 Dec 2025 13:51:10 +0000 (UTC) Received: from thuth-p1g4.redhat.com (unknown [10.44.22.8]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 06C73180035A; Mon, 8 Dec 2025 13:51:03 +0000 (UTC) From: Thomas Huth To: Peter Xu , Fabiano Rosas , qemu-devel@nongnu.org, Kevin Wolf Cc: qemu-block@nongnu.org, Eric Blake , Stefan Hajnoczi , Paolo Bonzini , =?UTF-8?q?Daniel=20P=20=2E=20Berrang=C3=A9?= , Markus Armbruster , Peter Maydell , =?UTF-8?q?Alex=20Benn=C3=A9e?= Subject: [PATCH] migration: Fix a possible crash when halting a guest during migration Date: Mon, 8 Dec 2025 14:51:01 +0100 Message-ID: <20251208135101.271417-1-thuth@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Received-SPF: pass client-ip=170.10.133.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: 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 From: Thomas Huth When shutting down a guest that is currently in progress of being migrated, there is a chance that QEMU might crash during bdrv_delete(). The backtrace looks like this: Thread 74 "mig/src/main" received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x3f7de7fc8c0 (LWP 2161436)] 0x000002aa00664012 in bdrv_delete (bs=0x2aa00f875c0) at ../../devel/qemu/block.c:5560 5560 QTAILQ_REMOVE(&graph_bdrv_states, bs, node_list); (gdb) bt #0 0x000002aa00664012 in bdrv_delete (bs=0x2aa00f875c0) at ../../devel/qemu/block.c:5560 #1 bdrv_unref (bs=0x2aa00f875c0) at ../../devel/qemu/block.c:7170 Backtrace stopped: Cannot access memory at address 0x3f7de7f83e0 The problem is apparently that the migration thread is still active (migration_shutdown() only asks it to stop the current migration, but does not wait for it to finish), while the main thread continues to bdrv_close_all() that will destroy all block drivers. So the two threads are racing here for the destruction of the migration-related block drivers. I was able to bisect the problem and the race has apparently been introduced by commit c2a189976e211c9ff782 ("migration/block-active: Remove global active flag"), so reverting it might be an option as well, but waiting for the migration thread to finish before continuing with the further clean-ups during shutdown seems less intrusive. Note: I used the Claude AI assistant for analyzing the crash, and it came up with the idea of waiting for the migration thread to finish in migration_shutdown() before proceeding with the further clean-up, but the patch itself has been 100% written by myself. Fixes: c2a189976e ("migration/block-active: Remove global active flag") Signed-off-by: Thomas Huth --- migration/migration.c | 24 ++++++++++++++++++------ 1 file changed, 18 insertions(+), 6 deletions(-) diff --git a/migration/migration.c b/migration/migration.c index b316ee01ab2..6f4bb6d8438 100644 --- a/migration/migration.c +++ b/migration/migration.c @@ -380,6 +380,16 @@ void migration_bh_schedule(QEMUBHFunc *cb, void *opaque) qemu_bh_schedule(bh); } +static void migration_thread_join(MigrationState *s) +{ + if (s && s->migration_thread_running) { + bql_unlock(); + qemu_thread_join(&s->thread); + s->migration_thread_running = false; + bql_lock(); + } +} + void migration_shutdown(void) { /* @@ -393,6 +403,13 @@ void migration_shutdown(void) * stop the migration using this structure */ migration_cancel(); + /* + * Wait for migration thread to finish to prevent a possible race where + * the migration thread is still running and accessing host block drivers + * while the main cleanup proceeds to remove them in bdrv_close_all() + * later. + */ + migration_thread_join(migrate_get_current()); object_unref(OBJECT(current_migration)); /* @@ -1499,12 +1516,7 @@ static void migration_cleanup(MigrationState *s) close_return_path_on_source(s); - if (s->migration_thread_running) { - bql_unlock(); - qemu_thread_join(&s->thread); - s->migration_thread_running = false; - bql_lock(); - } + migration_thread_join(s); WITH_QEMU_LOCK_GUARD(&s->qemu_file_lock) { /* -- 2.52.0