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Thu, 18 Nov 2021 08:51:27 -0500 X-MC-Unique: aYenik42OoGv0kL7iGH4Dg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1D71E19251A4; Thu, 18 Nov 2021 13:51:26 +0000 (UTC) Received: from [10.39.192.245] (unknown [10.39.192.245]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5D03960BF1; Thu, 18 Nov 2021 13:50:50 +0000 (UTC) Message-ID: Date: Thu, 18 Nov 2021 14:50:49 +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 00/25] block layer: split block APIs in global state and I/O To: Hanna Reitz , Emanuele Giuseppe Esposito , qemu-block@nongnu.org References: <20211025101735.2060852-1-eesposit@redhat.com> <93821bd8-2ac0-a19e-7029-900e6a6d9be1@redhat.com> From: Paolo Bonzini In-Reply-To: <93821bd8-2ac0-a19e-7029-900e6a6d9be1@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=pbonzini@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=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -45 X-Spam_score: -4.6 X-Spam_bar: ---- X-Spam_report: (-4.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.698, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-1.084, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-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: , Cc: Kevin Wolf , Fam Zheng , Vladimir Sementsov-Ogievskiy , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , Eduardo Habkost , Juan Quintela , Eric Blake , Richard Henderson , qemu-devel@nongnu.org, Markus Armbruster , Stefan Hajnoczi , John Snow , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 11/15/21 17:03, Hanna Reitz wrote: > > I only really see four solutions for this: > (1) We somehow make the amend job run in the main context under the BQL > and have it prevent all concurrent I/O access (seems bad) > (2) We can make the permission functions part of the I/O path (seems > wrong and probably impossible?) > (3) We can drop the permissions update and permanently require the > permissions that we need when updating keys (I think this might break > existing use cases) > (4) We can acquire the BQL around the permission update call and perhaps > that works? > > I don’t know how (4) would work but it’s basically the only reasonable > solution I can come up with.  Would this be a way to call a BQL function > from an I/O function? I think that would deadlock: main I/O thread -------- ----- start bdrv_co_amend take BQL bdrv_drain ... hangs ... (2) is definitely wrong. (3) I have no idea. Would it be possible or meaningful to do the bdrv_child_refresh_perms in qmp_x_blockdev_amend? It seems that all users need it, and in general it seems weird to amend a qcow2 or luks header (and thus the meaning of parts of the file) while others can write to the same file. Paolo