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[82.29.237.198]) by smtp.gmail.com with ESMTPSA id r18sm27888520wrt.96.2021.07.21.11.00.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Jul 2021 11:00:26 -0700 (PDT) Date: Wed, 21 Jul 2021 19:00:24 +0100 From: "Dr. David Alan Gilbert" To: Peter Xu Subject: Re: [PATCH 2/5] migration: Shutdown src in await_return_path_close_on_source() Message-ID: References: <20210721012134.792845-1-peterx@redhat.com> <20210721012134.792845-3-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.459, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: Peter Maydell , Lukas Straub , Leonardo Bras Soares Passos , qemu-devel@nongnu.org, Juan Quintela Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Peter Xu (peterx@redhat.com) wrote: > On Wed, Jul 21, 2021 at 10:55:00AM +0100, Dr. David Alan Gilbert wrote: > > * Peter Xu (peterx@redhat.com) wrote: > > > We have a logic in await_return_path_close_on_source() that we will explicitly > > > shutdown the socket when migration encounters errors. However it could be racy > > > because from_dst_file could have been reset right after checking it but before > > > passing it to qemu_file_shutdown() by the rp_thread. > > > > > > Fix it by shutdown() on the src file instead. Since they must be a pair of > > > qemu files, shutdown on either of them will work the same. > > > > > > Since at it, drop the check for from_dst_file directly, which makes the > > > behavior even more predictable. > > > > So while the existing code maybe racy, I'm not sure that this change > > keeps the semantics; the channel may well have dup()'d the fd's for the > > two directions, and I'm not convinced that a shutdown() on one will > > necessarily impact the other; and if the shutdown doesn't happen the > > rp_thread might not exit, and we might block on the koin. > > dup() seems fine as long as the backend file/socket is the same; but I get the > point here, e.g., potentially from/to channels can indeed be different ones. > > > > > Why don't we solve this a different way - how about we move the: > > ms->rp_state.from_dst_file = NULL; > > qemu_fclose(rp); > > > > out of the source_return_path_thread and put it in > > await_return_path_close_on_source, immediately after the join? > > Then we *know* that the the rp thread isn't messing with it. > > Yes that looks working for this special case of when rp_thread quits. > > It's just that it'll make things a bit more complicated, previously > from_dst_file is only reset in rp_thread (for either paused or completed > migration), now we handle "migration completes" a bit different. > > This also reminded me that maybe we can also use the qemu_file_lock mutex as we > use to try protect access to to_dst_file, because I think from_dst_file is > potentially racy in the same way. Even if we moved the reset to migration > thread, we still have e.g. migrate_fd_cancel() calling: > > if (s->rp_state.from_dst_file) { > qemu_file_shutdown(s->rp_state.from_dst_file); > } > > I think that is also racy too when running in the main thread, as either the > migration thread or rp_thread could have reset it right after the check. > > So.. maybe I start to use the qemu_file_lock to cover from_dst_file cases too? > Then I'll cover at leasd both migrate_fd_cancel() and this case. Yes, I think that's best; but try to do as little as possible with the lock held. Dave > Thanks, > > -- > Peter Xu > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK