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dkim=none (message not signed) header.d=none;nongnu.org; dmarc=none action=none header.from=virtuozzo.com; Received: from AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) by AS8PR08MB6904.eurprd08.prod.outlook.com (2603:10a6:20b:394::23) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4150.27; Wed, 26 May 2021 17:18:27 +0000 Received: from AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133]) by AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133%8]) with mapi id 15.20.4150.027; Wed, 26 May 2021 17:18:27 +0000 Subject: Re: [PATCH v2 7/7] block-copy: protect BlockCopyState .method fields To: Paolo Bonzini , Emanuele Giuseppe Esposito , qemu-block@nongnu.org Cc: John Snow , Kevin Wolf , Max Reitz , Stefan Hajnoczi , qemu-devel@nongnu.org References: <20210518100757.31243-1-eesposit@redhat.com> <20210518100757.31243-8-eesposit@redhat.com> <404e2891-9c03-bc7d-2c69-a572422d7804@virtuozzo.com> <4403ee17-0c12-c545-7f64-683bb9057b91@redhat.com> From: Vladimir Sementsov-Ogievskiy Message-ID: <47ae4243-5a4b-e3a5-44fb-48c689f8bc77@virtuozzo.com> Date: Wed, 26 May 2021 20:18:25 +0300 User-Agent: Mozilla/5.0 (X11; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR04-HE1-obe.outbound.protection.outlook.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 26.05.2021 17:48, Paolo Bonzini wrote: > On 25/05/21 13:00, Vladimir Sementsov-Ogievskiy wrote: >> >> Hmm. OK, let me think: >> >> First look at block_copy_do_copy(). It's called only from block_copy_task_entry. block_copy_task_entry() has mutex-critical-section anyway around handling return value. That means that we can simply move s->method modification logic to this already existing critical section. >> >> Next, block_copy_chunk_size() is called only from block_copy_task_create(), where we should have critical section too. > > block_copy_do_copy would have to release the mutex around the reads/writes (including the bdrv_co_pwrite_zeroes that has nothing to do with s->method). > > There's also the "goto out" if the copy-range operation succeeds, which makes things a bit more complicated.  The goto suggests using QEMU_WITH_LOCK_GUARD, but that doesn't work too well either, because there are two accesses (one before the bdrv_co_copy_range and one after). Hmm, this "goto out" makes no sense actually, and should be removed.. It's a mistake or a kind of forgotten thing to refactor after some changes. > > So I understand why you want to avoid atomics and I agree that they are more complicated than the other solutions, on the other hand I think this patch is the simplest code. > I think it's better to pass s->method as parameter to block_copy_do_copy. Then block_copy_do_copy() doesn't need any kind of synchronization. In block_copy_task_entry() we'll have the whole logic of handling result of block_copy_do_copy (including changing of s->method). And then, only one question is: before calling block_copy_do_copy() we should get s->method. Either by atomic operation or under separate critical section.. I'd be OK either way. It's actually the original idea of block_copy_do_copy() function: do only simple copy, don't interact with the state. It's a kind of wrapper on top of bdrv_co. So, actually updating s->use_copy_range here was a bad idea. Note also the comment above block_copy_do_copy(): " No sync here: nor bitmap neighter intersecting requests handling, only copy." Somehow, the comment conflicts with introducing synchronization primitives inside the function, although it was about another things :)) -- Best regards, Vladimir