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[109.164.254.58]) by smtp.gmail.com with ESMTPSA id f4-20020a1c6a04000000b003a54f1d007csm12844482wmc.10.2022.08.16.07.23.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 16 Aug 2022 07:23:55 -0700 (PDT) Message-ID: <0633bb24-97b7-c99d-4dd4-e92c3f76df73@redhat.com> Date: Tue, 16 Aug 2022 16:23:54 +0200 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 v10 10/21] block/mirror.c: use of job helpers in drivers to avoid TOC/TOU Content-Language: en-US To: Kevin Wolf Cc: qemu-block@nongnu.org, Hanna Reitz , Paolo Bonzini , John Snow , Vladimir Sementsov-Ogievskiy , Wen Congyang , Xie Changlong , Markus Armbruster , Stefan Hajnoczi , Fam Zheng , qemu-devel@nongnu.org, Vladimir Sementsov-Ogievskiy References: <20220725073855.76049-1-eesposit@redhat.com> <20220725073855.76049-11-eesposit@redhat.com> From: Emanuele Giuseppe Esposito In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=eesposit@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, 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, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 04/08/2022 um 18:35 schrieb Kevin Wolf: > Am 25.07.2022 um 09:38 hat Emanuele Giuseppe Esposito geschrieben: >> Once job lock is used and aiocontext is removed, mirror has >> to perform job operations under the same critical section, >> using the helpers prepared in previous commit. >> >> Note: at this stage, job_{lock/unlock} and job lock guard macros >> are *nop*. >> >> Reviewed-by: Vladimir Sementsov-Ogievskiy >> Reviewed-by: Stefan Hajnoczi >> Signed-off-by: Emanuele Giuseppe Esposito > > Can you explain in the commit message what the TOC/TOU case is that this > patch is addressing? It's not obvious to me why you picked exactly these > places to add locking. Well after thinking about it, mentioning TOC/TOU doesn't really make sense here. I'll remove the "to avoid TOC/TOU" in the title. > >> diff --git a/block/mirror.c b/block/mirror.c >> index d8ecb9efa2..b38676e19d 100644 >> --- a/block/mirror.c >> +++ b/block/mirror.c >> @@ -654,9 +654,13 @@ static int mirror_exit_common(Job *job) >> BlockDriverState *target_bs; >> BlockDriverState *mirror_top_bs; >> Error *local_err = NULL; >> - bool abort = job->ret < 0; >> + bool abort; >> int ret = 0; >> >> + WITH_JOB_LOCK_GUARD() { >> + abort = job->ret < 0; >> + } > > This is the most mysterious hunk to me. The only thing that should > modify job->ret is the caller of this function anyway, but let's assume > for a moment that another thread could write to it. > > Then why is it only important that we hold the lock when we're reading > the value, but not any more when we are actually using it? And what is > the TOC/TOU that this fixes? No TOC/TOU, no sense for this fix too. I'll remove this hunk too. Emanuele > > Kevin >