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([2a02:8071:5055:3f20:7ad9:a400:6d51:83e6]) by smtp.gmail.com with ESMTPSA id n66sm5384087wmn.2.2021.11.04.06.34.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 04 Nov 2021 06:34:42 -0700 (PDT) Message-ID: <67f567b7-8d7a-3990-fced-93a082c810b7@redhat.com> Date: Thu, 4 Nov 2021 14:34:41 +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 0/7] block: Attempt on fixing 030-reported errors To: Kevin Wolf References: <20211104103849.46855-1-hreitz@redhat.com> From: Hanna Reitz In-Reply-To: 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: -54 X-Spam_score: -5.5 X-Spam_bar: ----- X-Spam_report: (-5.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.648, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-2.093, 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: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 04.11.21 12:58, Kevin Wolf wrote: > Am 04.11.2021 um 11:38 hat Hanna Reitz geschrieben: >> (2A) bdrv_replace_child_noperm() should immediately set bs->file or >> bs->backing to NULL when it sets bs->{file,backing}->bs to NULL. >> It should also immediately remove any BdrvChild with .bs == NULL >> from the parent’s BDS.children list. >> Implemented in patches 2 through 6. >> >> (2B) Alternatively, we could always keep the whole subgraph drained >> while we manipulate it. Then, the bdrv_parent_drained_end_single() >> in bdrv_replace_child_noperm() wouldn’t do anything. >> To fix 030, we would need to add a drained section to >> stream_prepare(): Namely we’d need to drain the subgraph below the >> COR filter node. >> This would be a much simpler solution, but I don’t feel like it’s >> the right one. >> As you can see, I’m not sure which of 2A or 2B is the right solution. I >> decided to investigate both: 2A was much more complicated, but seemed >> like the right thing to do; 2B is much simpler, but doesn’t feel as >> right. Therefore, I decided to go with 2A in this first version of this >> series. > I haven't looked at the patches yet, but if I understand correctly the > choice you're presenting here is between protecting code from accessing > invalid state and not creating the invalid state in the first place. Yes, that’s right. > I agree that the latter is preferable as long as it doesn't make things > so complicated that we would be willing to accept the higher risk of > breakage in the former. No, I don’t think it’s too complicated.  Just not as sample as a drained_begin + drained_end. > If it's doable in five patches, it's probably > not complicated enough to make such compromises. Without the clean-up patches that are patches 3 and 4, it would be doable in even fewer patches. :) Hanna