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Tue, 17 Sep 2024 17:41:32 +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 wPwBFEy/6Wa/eQAAD6G6ig (envelope-from ); Tue, 17 Sep 2024 17:41:32 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Peter Maydell , qemu-stable@nongnu.org Subject: Re: [PATCH 2/2] migration/multifd: Fix rb->receivedmap cleanup race In-Reply-To: References: <20240913220542.18305-1-farosas@suse.de> <20240913220542.18305-3-farosas@suse.de> Date: Tue, 17 Sep 2024 14:41:30 -0300 Message-ID: <87frpyyy7p.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_THREE(0.00)[4]; RCVD_COUNT_TWO(0.00)[2]; FUZZY_BLOCKED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,suse.de:mid] Received-SPF: pass client-ip=195.135.223.130; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 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_MED=-2.3, 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: > On Fri, Sep 13, 2024 at 07:05:42PM -0300, Fabiano Rosas wrote: >> Fix a segmentation fault in multifd when rb->receivedmap is cleared >> too early. >> >> After commit 5ef7e26bdb ("migration/multifd: solve zero page causing >> multiple page faults"), multifd started using the rb->receivedmap >> bitmap, which belongs to ram.c and is initialized and *freed* from the >> ram SaveVMHandlers. >> >> Multifd threads are live until migration_incoming_state_destroy(), >> which is called after qemu_loadvm_state_cleanup(), leading to a crash >> when accessing rb->receivedmap. >> >> process_incoming_migration_co() ... >> qemu_loadvm_state() multifd_nocomp_recv() >> qemu_loadvm_state_cleanup() ramblock_recv_bitmap_set_offset() >> rb->receivedmap = NULL set_bit_atomic(..., rb->receivedmap) >> ... >> migration_incoming_state_destroy() >> multifd_recv_cleanup() >> multifd_recv_terminate_threads(NULL) >> >> Move the loadvm cleanup into migration_incoming_state_destroy(), after >> multifd_recv_cleanup() to ensure multifd thread have already exited >> when rb->receivedmap is cleared. >> >> The have_listen_thread logic can now be removed because its purpose >> was to delay cleanup until postcopy_ram_listen_thread() had finished. >> >> CC: qemu-stable@nongnu.org >> Fixes: 5ef7e26bdb ("migration/multifd: solve zero page causing multiple page faults") >> Signed-off-by: Fabiano Rosas >> --- >> migration/migration.c | 1 + >> migration/migration.h | 1 - >> migration/savevm.c | 9 --------- >> 3 files changed, 1 insertion(+), 10 deletions(-) >> >> diff --git a/migration/migration.c b/migration/migration.c >> index 3dea06d577..b190a574b1 100644 >> --- a/migration/migration.c >> +++ b/migration/migration.c >> @@ -378,6 +378,7 @@ void migration_incoming_state_destroy(void) >> struct MigrationIncomingState *mis = migration_incoming_get_current(); >> >> multifd_recv_cleanup(); >> + qemu_loadvm_state_cleanup(); >> >> if (mis->to_src_file) { >> /* Tell source that we are done */ >> diff --git a/migration/migration.h b/migration/migration.h >> index 38aa1402d5..20b0a5b66e 100644 >> --- a/migration/migration.h >> +++ b/migration/migration.h >> @@ -101,7 +101,6 @@ struct MigrationIncomingState { >> /* Set this when we want the fault thread to quit */ >> bool fault_thread_quit; >> >> - bool have_listen_thread; >> QemuThread listen_thread; >> >> /* For the kernel to send us notifications */ >> diff --git a/migration/savevm.c b/migration/savevm.c >> index d0759694fd..532ee5e4b0 100644 >> --- a/migration/savevm.c >> +++ b/migration/savevm.c >> @@ -2076,10 +2076,8 @@ static void *postcopy_ram_listen_thread(void *opaque) >> * got a bad migration state). >> */ >> migration_incoming_state_destroy(); >> - qemu_loadvm_state_cleanup(); >> >> rcu_unregister_thread(); >> - mis->have_listen_thread = false; >> postcopy_state_set(POSTCOPY_INCOMING_END); >> >> object_unref(OBJECT(migr)); >> @@ -2130,7 +2128,6 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis) >> return -1; >> } >> >> - mis->have_listen_thread = true; >> postcopy_thread_create(mis, &mis->listen_thread, "mig/dst/listen", >> postcopy_ram_listen_thread, QEMU_THREAD_DETACHED); >> trace_loadvm_postcopy_handle_listen("return"); >> @@ -2978,11 +2975,6 @@ int qemu_loadvm_state(QEMUFile *f) >> >> trace_qemu_loadvm_state_post_main(ret); >> >> - if (mis->have_listen_thread) { >> - /* Listen thread still going, can't clean up yet */ >> - return ret; >> - } > > Hmm, I wonder whether we would still need this. IIUC it's not only about > cleanup, but also that when postcopy is involved, dst QEMU postpones doing > any of the rest in the qemu_loadvm_state_main() call. > > E.g. cpu put, aka, cpu_synchronize_all_post_init(), is also done in > loadvm_postcopy_handle_run_bh() later. > > IOW, I'd then expect when this patch applied we'll put cpu twice? > > I think the should_send_vmdesc() part is fine, as it returns false for > postcopy anyway. However not sure on the cpu post_init above. I'm not sure either, but there's several ioctls in there, so it's probably better to skip them. I'll keep the have_listen and adjust the comment.