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Tue, 08 Oct 2019 22:36:46 -0700 (PDT) X-Google-Smtp-Source: APXvYqy8KTZWX0Ozq6dUDerIv1aAVQjlo2yzMpqJEcrA18NkQ6ltW6VypGJ8sYqE58jZGqLQ7axa0g== X-Received: by 2002:a63:1f25:: with SMTP id f37mr2467151pgf.50.1570599405948; Tue, 08 Oct 2019 22:36:45 -0700 (PDT) Received: from xz-x1 ([209.132.188.80]) by smtp.gmail.com with ESMTPSA id n3sm1010533pff.102.2019.10.08.22.36.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 08 Oct 2019 22:36:44 -0700 (PDT) Date: Wed, 9 Oct 2019 13:36:34 +0800 From: Peter Xu To: Wei Yang Subject: Re: [PATCH 3/3] migration/postcopy: handle POSTCOPY_INCOMING_RUNNING corner case properly Message-ID: <20191009053633.GA1039@xz-x1> References: <20191001100122.17730-1-richardw.yang@linux.intel.com> <20191001100122.17730-4-richardw.yang@linux.intel.com> <20191008164046.GF3441@work-vm> <20191009010204.GC26203@richard> <20191009041225.GF10750@xz-x1> <20191009050756.GA9616@richard> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20191009050756.GA9616@richard> User-Agent: Mutt/1.11.4 (2019-03-13) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 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: qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , quintela@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Oct 09, 2019 at 01:07:56PM +0800, Wei Yang wrote: > On Wed, Oct 09, 2019 at 12:12:25PM +0800, Peter Xu wrote: > >On Wed, Oct 09, 2019 at 09:02:04AM +0800, Wei Yang wrote: > >> On Tue, Oct 08, 2019 at 05:40:46PM +0100, Dr. David Alan Gilbert wrote: > >> >* Wei Yang (richardw.yang@linux.intel.com) wrote: > >> >> Currently, we set PostcopyState blindly to RUNNING, even we found the > >> >> previous state is not LISTENING. This will lead to a corner case. > >> >> > >> >> First let's look at the code flow: > >> >> > >> >> qemu_loadvm_state_main() > >> >> ret = loadvm_process_command() > >> >> loadvm_postcopy_handle_run() > >> >> return -1; > >> >> if (ret < 0) { > >> >> if (postcopy_state_get() == POSTCOPY_INCOMING_RUNNING) > >> >> ... > >> >> } > >> >> > >> >> From above snippet, the corner case is loadvm_postcopy_handle_run() > >> >> always sets state to RUNNING. And then it checks the previous state. If > >> >> the previous state is not LISTENING, it will return -1. But at this > >> >> moment, PostcopyState is already been set to RUNNING. > >> >> > >> >> Then ret is checked in qemu_loadvm_state_main(), when it is -1 > >> >> PostcopyState is checked. Current logic would pause postcopy and retry > >> >> if PostcopyState is RUNNING. This is not what we expect, because > >> >> postcopy is not active yet. > >> >> > >> >> This patch makes sure state is set to RUNNING only previous state is > >> >> LISTENING by introducing an old_state parameter in postcopy_state_set(). > >> >> New state only would be set when current state equals to old_state. > >> >> > >> >> Signed-off-by: Wei Yang > >> > > >> >OK, it's a shame to use a pointer there, but it works. > >> > >> You mean second parameter of postcopy_state_set()? > >> > >> I don't have a better idea. Or we introduce a new state > >> POSTCOPY_INCOMING_NOCHECK. Do you feel better with this? > > > >Maybe simply fix loadvm_postcopy_handle_run() to set the state after > >the POSTCOPY_INCOMING_LISTENING check? > > > > Set state back to ps if ps is not POSTCOPY_INCOMING_LISTENING? > > Sounds like another option. Even simpler? ps = postcopy_state_get(); if (ps != INCOMING) return -1; postcopy_state_set(RUNNING); Thanks, -- Peter Xu