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bh=GGFmnCZBV71cdeNB8RS4f6ngdrPmRvsBOxFBjUsklWM=; b=IoaUcp3xEMGTy1YszAgi0I13UNwLsVp8c3MBXLrA8eqGtDq/HqoVquvIxjHo1MIwifZzzH pzADhZkGlLQo9PTW1eN1n8TdnyoNLfouEs9UOR/sAyVPNl8TyFJ9M2D9jA4OUn1PP3c18W 5dYKI/SCIbDVs8xLux25Dj2vVpIrxRQ= X-MC-Unique: ls-2rlfyPbS-Xuy0FiEcQw-1 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 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; 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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1711461375580100003 Content-Type: text/plain; charset="utf-8" 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 qc= ow2 > ./qemu-system-x86_64 --qmp stdio \ > --blockdev qcow2,node-name=3Dnode0,file.driver=3Dfile,file.filename=3D/tm= p/top.qcow2 \ > < {"execute": "qmp_capabilities"} > {"execute": "block-stream", "arguments": { "job-id": "stream0", "device":= "node0" } } > {"execute": "quit"} > EOF [0]: > #0 bdrv_replace_child_tran (child=3D..., new_bs=3D..., tran=3D...) > #1 bdrv_replace_node_noperm (from=3D..., to=3D..., auto_skip=3D..., tran= =3D..., errp=3D...) > #2 bdrv_replace_node_common (from=3D..., to=3D..., auto_skip=3D..., deta= ch_subchain=3D..., errp=3D...) > #3 bdrv_drop_filter (bs=3D..., errp=3D...) > #4 bdrv_cor_filter_drop (cor_filter_bs=3D...) > #5 stream_prepare (job=3D...) > #6 job_prepare_locked (job=3D...) > #7 job_txn_apply_locked (fn=3D..., job=3D...) > #8 job_do_finalize_locked (job=3D...) > #9 job_exit (opaque=3D...) > #10 aio_bh_poll (ctx=3D...) > #11 aio_poll (ctx=3D..., blocking=3D...) > #12 bdrv_poll_co (s=3D...) > #13 bdrv_flush (bs=3D...) > #14 bdrv_flush_all () > #15 do_vm_stop (state=3D..., send_stop=3D...) > #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() =3D=3D qemu_get_aio_context()); =20 + old_bs =3D 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 =3D=3D BDRV_NEXT_BACKEND_ROOTS) { BlockBackend *old_blk =3D it->blk; =20 - old_bs =3D old_blk ? blk_bs(old_blk) : NULL; - do { it->blk =3D blk_all_next(it->blk); bs =3D 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 =3D bs; return bs; } it->phase =3D BDRV_NEXT_MONITOR_OWNED; - } else { - old_bs =3D it->bs; } =20 /* Then return the monitor-owned BDSes without a BB attached. Ignore a= ll --=20 2.44.0