From patchwork Tue Mar 26 13:54:38 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kevin Wolf X-Patchwork-Id: 13604232 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 78081C6FD1F for ; Tue, 26 Mar 2024 13:56:13 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rp7GT-0004BD-WA; Tue, 26 Mar 2024 09:54:58 -0400 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 1rp7GR-00049J-IC for qemu-devel@nongnu.org; Tue, 26 Mar 2024 09:54:55 -0400 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 1rp7GP-0001Ob-U6 for qemu-devel@nongnu.org; Tue, 26 Mar 2024 09:54:55 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1711461293; 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: in-reply-to:in-reply-to:references:references; bh=GGFmnCZBV71cdeNB8RS4f6ngdrPmRvsBOxFBjUsklWM=; b=IoaUcp3xEMGTy1YszAgi0I13UNwLsVp8c3MBXLrA8eqGtDq/HqoVquvIxjHo1MIwifZzzH pzADhZkGlLQo9PTW1eN1n8TdnyoNLfouEs9UOR/sAyVPNl8TyFJ9M2D9jA4OUn1PP3c18W 5dYKI/SCIbDVs8xLux25Dj2vVpIrxRQ= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-590-ls-2rlfyPbS-Xuy0FiEcQw-1; Tue, 26 Mar 2024 09:54:51 -0400 X-MC-Unique: ls-2rlfyPbS-Xuy0FiEcQw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7C0688B5AE9; Tue, 26 Mar 2024 13:54:51 +0000 (UTC) Received: from merkur.fritz.box (unknown [10.39.193.60]) by smtp.corp.redhat.com (Postfix) with ESMTP id C5CA61121312; Tue, 26 Mar 2024 13:54:50 +0000 (UTC) From: Kevin Wolf To: qemu-block@nongnu.org Cc: kwolf@redhat.com, peter.maydell@linaro.org, qemu-devel@nongnu.org Subject: [PULL 4/6] block-backend: fix edge case in bdrv_next() where BDS associated to BB changes Date: Tue, 26 Mar 2024 14:54:38 +0100 Message-ID: <20240326135440.421609-5-kwolf@redhat.com> In-Reply-To: <20240326135440.421609-1-kwolf@redhat.com> References: <20240326135440.421609-1-kwolf@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.088, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, 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: 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: Fiona Ebner The old_bs variable in bdrv_next() is currently determined by looking at the old block backend. However, if the block graph changes before the next bdrv_next() call, it might be that the associated BDS is not the same that was referenced previously. In that case, the wrong BDS is unreferenced, leading to an assertion failure later: > bdrv_unref: Assertion `bs->refcnt > 0' failed. In particular, this can happen in the context of bdrv_flush_all(), when polling for bdrv_co_flush() in the generated co-wrapper leads to a graph change (for example with a stream block job [0]). A racy reproducer: > #!/bin/bash > rm -f /tmp/backing.qcow2 > rm -f /tmp/top.qcow2 > ./qemu-img create /tmp/backing.qcow2 -f qcow2 64M > ./qemu-io -c "write -P42 0x0 0x1" /tmp/backing.qcow2 > ./qemu-img create /tmp/top.qcow2 -f qcow2 64M -b /tmp/backing.qcow2 -F qcow2 > ./qemu-system-x86_64 --qmp stdio \ > --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/top.qcow2 \ > < {"execute": "qmp_capabilities"} > {"execute": "block-stream", "arguments": { "job-id": "stream0", "device": "node0" } } > {"execute": "quit"} > EOF [0]: > #0 bdrv_replace_child_tran (child=..., new_bs=..., tran=...) > #1 bdrv_replace_node_noperm (from=..., to=..., auto_skip=..., tran=..., errp=...) > #2 bdrv_replace_node_common (from=..., to=..., auto_skip=..., detach_subchain=..., errp=...) > #3 bdrv_drop_filter (bs=..., errp=...) > #4 bdrv_cor_filter_drop (cor_filter_bs=...) > #5 stream_prepare (job=...) > #6 job_prepare_locked (job=...) > #7 job_txn_apply_locked (fn=..., job=...) > #8 job_do_finalize_locked (job=...) > #9 job_exit (opaque=...) > #10 aio_bh_poll (ctx=...) > #11 aio_poll (ctx=..., blocking=...) > #12 bdrv_poll_co (s=...) > #13 bdrv_flush (bs=...) > #14 bdrv_flush_all () > #15 do_vm_stop (state=..., send_stop=...) > #16 vm_shutdown () Signed-off-by: Fiona Ebner Message-ID: <20240322095009.346989-3-f.ebner@proxmox.com> Reviewed-by: Kevin Wolf Reviewed-by: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- block/block-backend.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/block/block-backend.c b/block/block-backend.c index 9c4de79e6b..28af1eb17a 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -599,14 +599,14 @@ BlockDriverState *bdrv_next(BdrvNextIterator *it) /* Must be called from the main loop */ assert(qemu_get_current_aio_context() == qemu_get_aio_context()); + old_bs = it->bs; + /* First, return all root nodes of BlockBackends. In order to avoid * returning a BDS twice when multiple BBs refer to it, we only return it * if the BB is the first one in the parent list of the BDS. */ if (it->phase == BDRV_NEXT_BACKEND_ROOTS) { BlockBackend *old_blk = it->blk; - old_bs = old_blk ? blk_bs(old_blk) : NULL; - do { it->blk = blk_all_next(it->blk); bs = it->blk ? blk_bs(it->blk) : NULL; @@ -620,11 +620,10 @@ BlockDriverState *bdrv_next(BdrvNextIterator *it) if (bs) { bdrv_ref(bs); bdrv_unref(old_bs); + it->bs = bs; return bs; } it->phase = BDRV_NEXT_MONITOR_OWNED; - } else { - old_bs = it->bs; } /* Then return the monitor-owned BDSes without a BB attached. Ignore all