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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id t23sm742260wmj.31.2021.05.07.06.45.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 07 May 2021 06:45:46 -0700 (PDT) To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20210506090621.11848-1-vsementsov@virtuozzo.com> <20210506090621.11848-6-vsementsov@virtuozzo.com> From: Paolo Bonzini Subject: Re: [PATCH v3 5/8] block/write-threshold: don't use aio context lock Message-ID: <700a25a7-c2aa-51fa-45c8-e97932c07c2e@redhat.com> Date: Fri, 7 May 2021 15:45:45 +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: <20210506090621.11848-6-vsementsov@virtuozzo.com> 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-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=pbonzini@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: fam@euphon.net, kwolf@redhat.com, eesposit@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 06/05/21 11:06, Vladimir Sementsov-Ogievskiy wrote: > void bdrv_write_threshold_check_write(BlockDriverState *bs, int64_t offset, > int64_t bytes) > { > int64_t end = offset + bytes; > - uint64_t wtr = bs->write_threshold_offset; > + uint64_t wtr; > > - if (wtr > 0 && end > wtr) { > - qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr); > +retry: > + wtr = bdrv_write_threshold_get(bs); > + if (wtr == 0 || wtr >= end) { > + return; > + } > > - /* autodisable to avoid flooding the monitor */ > - bdrv_write_threshold_set(bs, 0); > + /* autodisable to avoid flooding the monitor */ > + if (qatomic_cmpxchg(&bs->write_threshold_offset, wtr, 0) != wtr) { > + /* bs->write_threshold_offset changed in parallel */ > + goto retry; > } > + > + /* We have cleared bs->write_threshold_offset, so let's send event */ > + qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr); > } > This has the problem that 64-bit atomics are not always possible on 32-bit builds. We can use a spinlock (and probably just drop this patch for now). Paolo