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Tue, 29 Oct 2024 18:48:07 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id wP4bNectIWdwTAAAD6G6ig (envelope-from ); Tue, 29 Oct 2024 18:48:07 +0000 From: Fabiano Rosas To: Peter Xu , qemu-devel@nongnu.org Cc: Avihai Horon , =?utf-8?Q?C=C3=A9dric?= Le Goater , peterx@redhat.com, Alex Williamson Subject: Re: [PATCH v3 1/8] migration: Take migration object refcount earlier for threads In-Reply-To: <20241024213056.1395400-2-peterx@redhat.com> References: <20241024213056.1395400-1-peterx@redhat.com> <20241024213056.1395400-2-peterx@redhat.com> Date: Tue, 29 Oct 2024 15:48:05 -0300 Message-ID: <875xpad9ui.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_FIVE(0.00)[6]; RCVD_COUNT_TWO(0.00)[2]; FUZZY_BLOCKED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo, suse.de:mid, suse.de:email] Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:1; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, 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: 2.1.29 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org Peter Xu writes: > Both migration thread or background snapshot thread will take a refcount of > the migration object at the entrace of the thread function. > > That makes sense, because it protects the object from being freed by the > main thread in migration_shutdown() later, but it might still race with it > if the thread is scheduled too late. Consider the case right after > pthread_create() happened, VM shuts down with the object released, but > right after that the migration thread finally got created, referencing > MigrationState* in the opaque pointer which is already freed. > > The only 100% safe way to make sure it won't get freed is taking the > refcount right before the thread is created, meanwhile when BQL is held. > > Signed-off-by: Peter Xu Reviewed-by: Fabiano Rosas