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([2a02:8071:5055:3f20:7ad9:a400:6d51:83e6]) by smtp.gmail.com with ESMTPSA id v7sm5721716wrq.25.2021.11.12.06.40.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Nov 2021 06:40:41 -0800 (PST) Message-ID: <734073bb-80e6-3caf-d6b5-c8f2ade86044@redhat.com> Date: Fri, 12 Nov 2021 15:40:39 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v4 08/25] block: introduce assert_bdrv_graph_writable To: Emanuele Giuseppe Esposito , qemu-block@nongnu.org References: <20211025101735.2060852-1-eesposit@redhat.com> <20211025101735.2060852-9-eesposit@redhat.com> From: Hanna Reitz In-Reply-To: <20211025101735.2060852-9-eesposit@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=hreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -68 X-Spam_score: -6.9 X-Spam_bar: ------ X-Spam_report: (-6.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.699, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-3.449, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=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: , Cc: Kevin Wolf , Fam Zheng , Vladimir Sementsov-Ogievskiy , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , Eduardo Habkost , Juan Quintela , qemu-devel@nongnu.org, John Snow , Richard Henderson , Markus Armbruster , "Dr. David Alan Gilbert" , Stefan Hajnoczi , Paolo Bonzini , Eric Blake Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 25.10.21 12:17, Emanuele Giuseppe Esposito wrote: > We want to be sure that the functions that write the child and > parent list of a bs are under BQL and drain. > > BQL prevents from concurrent writings from the GS API, while > drains protect from I/O. > > TODO: drains are missing in some functions using this assert. > Therefore a proper assertion will fail. Because adding drains > requires additional discussions, they will be added in future > series. > > Signed-off-by: Emanuele Giuseppe Esposito > Reviewed-by: Paolo Bonzini > Reviewed-by: Stefan Hajnoczi > --- > block.c | 5 +++++ > block/io.c | 11 +++++++++++ > include/block/block_int-global-state.h | 10 +++++++++- > 3 files changed, 25 insertions(+), 1 deletion(-) > > diff --git a/block.c b/block.c > index 41c5883c5c..94bff5c757 100644 > --- a/block.c > +++ b/block.c > @@ -2734,12 +2734,14 @@ static void bdrv_replace_child_noperm(BdrvChild *child, > if (child->klass->detach) { > child->klass->detach(child); > } > + assert_bdrv_graph_writable(old_bs); > QLIST_REMOVE(child, next_parent); I think this belongs above the .detach() call (and the QLIST_REMOVE() belongs into the .detach() implementation, as done in https://lists.nongnu.org/archive/html/qemu-block/2021-11/msg00240.html, which has been merged to Kevin’s block branch). > } > > child->bs = new_bs; > > if (new_bs) { > + assert_bdrv_graph_writable(new_bs); > QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent); In both these places it’s a bit strange that the assertion is done on the child nodes.  The subgraph starting from them isn’t modified after all, so their subgraph technically doesn’t need to be writable.  I think a single assertion on the parent node would be preferable. I presume the problem with that is that we don’t have the parent node here?  Do we need a new BdrvChildClass method that performs this assertion on the parent node? > > /* > @@ -2940,6 +2942,7 @@ static int bdrv_attach_child_noperm(BlockDriverState *parent_bs, > return ret; > } > > + assert_bdrv_graph_writable(parent_bs); > QLIST_INSERT_HEAD(&parent_bs->children, *child, next); > /* > * child is removed in bdrv_attach_child_common_abort(), so don't care to > @@ -3140,6 +3143,7 @@ static void bdrv_unset_inherits_from(BlockDriverState *root, BdrvChild *child, > void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child) > { > assert(qemu_in_main_thread()); > + assert_bdrv_graph_writable(parent); It looks to me like we have this assertion mainly because bdrv_replace_child_noperm() doesn’t have a pointer to this parent node.  It’s a workaround, but we should have this in every path that eventually ends up at bdrv_replace_child_noperm(), and that seems rather difficult for the bdrv_replace_node() family of functions. That to me sounds like it’d be good to have this as a BdrvChildClass function. > if (child == NULL) { > return; > } > @@ -4903,6 +4907,7 @@ static void bdrv_remove_filter_or_cow_child_abort(void *opaque) > BdrvRemoveFilterOrCowChild *s = opaque; > BlockDriverState *parent_bs = s->child->opaque; > > + assert_bdrv_graph_writable(parent_bs); > QLIST_INSERT_HEAD(&parent_bs->children, s->child, next); > if (s->is_backing) { > parent_bs->backing = s->child; > diff --git a/block/io.c b/block/io.c > index f271ab3684..1c71e354d6 100644 > --- a/block/io.c > +++ b/block/io.c > @@ -740,6 +740,17 @@ void bdrv_drain_all(void) > bdrv_drain_all_end(); > } > > +void assert_bdrv_graph_writable(BlockDriverState *bs) > +{ > + /* > + * TODO: this function is incomplete. Because the users of this > + * assert lack the necessary drains, check only for BQL. > + * Once the necessary drains are added, > + * assert also for qatomic_read(&bs->quiesce_counter) > 0 > + */ > + assert(qemu_in_main_thread()); > +} > + > /** > * Remove an active request from the tracked requests list > * > diff --git a/include/block/block_int-global-state.h b/include/block/block_int-global-state.h > index d08e80222c..6bd7746409 100644 > --- a/include/block/block_int-global-state.h > +++ b/include/block/block_int-global-state.h > @@ -316,4 +316,12 @@ void bdrv_remove_aio_context_notifier(BlockDriverState *bs, > */ > void bdrv_drain_all_end_quiesce(BlockDriverState *bs); > > -#endif /* BLOCK_INT_GLOBAL_STATE*/ > +/** > + * Make sure that the function is either running under > + * drain and BQL. The latter protects from concurrent writings “either ... and” sounds wrong to me.  I’d drop the “either” or say “running under both drain and BQL”. Hanna > + * from the GS API, while the former prevents concurrent reads > + * from I/O. > + */ > +void assert_bdrv_graph_writable(BlockDriverState *bs); > + > +#endif /* BLOCK_INT_GLOBAL_STATE */