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([2a04:ee41:4:31cb:e591:1e1e:abde:a8f1]) by smtp.gmail.com with ESMTPSA id i25sm4269710eje.6.2021.05.14.14.53.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 May 2021 14:53:27 -0700 (PDT) Subject: Re: [PATCH 5/6] co-shared-resource: protect with a mutex To: Vladimir Sementsov-Ogievskiy , Stefan Hajnoczi References: <20210510085941.22769-1-eesposit@redhat.com> <20210510085941.22769-6-eesposit@redhat.com> <6d1e432e-f18a-39a4-0bb6-2a14347c2905@redhat.com> <6b9d7c37-aaf7-1745-260b-4cce8f0891ee@virtuozzo.com> <8008b39d-905c-3858-a96f-8609801a4ae0@redhat.com> <24be08c6-d1f1-802c-a045-3a5c3fe102b0@virtuozzo.com> From: Emanuele Giuseppe Esposito Message-ID: Date: Fri, 14 May 2021 23:53:26 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eesposit@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=WINDOWS-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=eesposit@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 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=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , qemu-block@nongnu.org, qemu-devel@nongnu.org, Max Reitz , Paolo Bonzini , John Snow Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" >>>>> we want to get from shres here, after possible call to >>>>> block_copy_task_shrink(), as task->bytes may be reduced. >>>> >>>> Ah right, I missed that. So I guess if we want the caller to protect >>>> co-shared-resource, get_from_shres stays where it is, and put_ >>>> instead can still go into task_end (with a boolean enabling it). >>> >>> honestly, I don't follow how it helps thread-safety >> >>  From my understanding, the whole point here is to have no lock in >> co-shared-resource but let the caller take care of it (block-copy). >> >> The above was just an idea on how to do it. > > But how moving co_put_to_shres() make it thread-safe? Nothing in > block-copy is thread-safe yet.. > Sorry this is my bad, I did not explain it properly. If you look closely at the diff I sent, there are locks in a similar way of my block-copy initial patch. So I am essentially assuming that block-copy has already locks, and moving co_put_to_shres in block_copy_task_end has the purpose of moving shres "in a function that has a critical section". >>>>>>> @@ -269,6 +270,7 @@ static void coroutine_fn >>>>>>> block_copy_task_end(BlockCopyTask *task, int ret) >>>>>>> bdrv_set_dirty_bitmap(task->s->copy_bitmap, >>>>>>> task->offset, task->bytes); >>>>>>> } >>>>>>> qemu_co_mutex_lock(&task->s->tasks_lock); ^^^ locks >>>>>>> + co_put_to_shres(task->s->mem, task->bytes); >>>>>>> task->s->in_flight_bytes -= task->bytes; >>>>>>> QLIST_REMOVE(task, list); >>>>>>> progress_set_remaining(task->s->progress, unlocks here (not shown in the diff) } Hopefully now it is clear. Apologies again for the confusion. Emanuele