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Fri, 11 Feb 2022 12:34:31 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.97]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2EE6B78DFE; Fri, 11 Feb 2022 12:34:20 +0000 (UTC) Date: Fri, 11 Feb 2022 13:34:18 +0100 From: Kevin Wolf To: Emanuele Giuseppe Esposito Subject: Re: [PATCH 3/6] block.c: bdrv_replace_child_noperm: first call ->attach(), and then add child Message-ID: References: <20220208153655.1251658-1-eesposit@redhat.com> <20220208153655.1251658-4-eesposit@redhat.com> MIME-Version: 1.0 In-Reply-To: <20220208153655.1251658-4-eesposit@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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: , Cc: Fam Zheng , Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, qemu-devel@nongnu.org, Hanna Reitz , Stefan Hajnoczi , Paolo Bonzini , John Snow Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 08.02.2022 um 16:36 hat Emanuele Giuseppe Esposito geschrieben: > Doing the opposite can make adding the child node to a non-drained node, > as apply_subtree_drain is only done in ->attach() and thus make > assert_bdrv_graph_writable fail. > > This can happen for example during a transaction rollback (test 245, > test_io_with_graph_changes): > 1. a node is removed from the graph, thus it is undrained > 2. then something happens, and we need to roll back the transactions > through tran_abort() > 3. at this point, the current code would first attach the undrained node > to the graph via QLIST_INSERT_HEAD, and then call ->attach() that > will take care of restoring the drain with apply_subtree_drain(), > leaving the node undrained between the two operations. > > Signed-off-by: Emanuele Giuseppe Esposito > --- > block.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/block.c b/block.c > index ec346a7e2e..08a6e3a4ef 100644 > --- a/block.c > +++ b/block.c > @@ -2872,8 +2872,6 @@ static void bdrv_replace_child_noperm(BdrvChild **childp, > } > > if (new_bs) { > - assert_bdrv_graph_writable(new_bs); > - QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent); > > /* > * Detaching the old node may have led to the new node's > @@ -2890,6 +2888,10 @@ static void bdrv_replace_child_noperm(BdrvChild **childp, > if (child->klass->attach) { > child->klass->attach(child); > } > + > + assert_bdrv_graph_writable(new_bs); > + QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent); > + > } Extra empty line. Looks good otherwise. Does this also mean that the order in bdrv_child_cb_attach/detach() is wrong? Or maybe adding a new node to bs->children is okay even when the child node isn't drained. Kevin