On Mon, Feb 05, 2024 at 01:10:14PM +0200, Avihai Horon wrote: > > On 05/02/2024 8:20, Peter Xu wrote: > > External email: Use caution opening links or attachments > > > > > > On Fri, Feb 02, 2024 at 04:11:28PM -0300, Fabiano Rosas wrote: > > > It is possible that one of the multifd channels fails to be created at > > > multifd_new_send_channel_async() while the rest of the channel > > > creation tasks are still in flight. > > > > > > This could lead to multifd_save_cleanup() executing the > > > qemu_thread_join() loop too early and not waiting for the threads > > > which haven't been created yet, leading to the freeing of resources > > > that the newly created threads will try to access and crash. > > > > > > Add a synchronization point after which there will be no attempts at > > > thread creation and therefore calling multifd_save_cleanup() past that > > > point will ensure it properly waits for the threads. > > > > > > A note about performance: Prior to this patch, if a channel took too > > > long to be established, other channels could finish connecting first > > > and already start taking load. Now we're bounded by the > > > slowest-connecting channel. > > Yes, I think this should (hopefully!) be fine. > > > > > Signed-off-by: Fabiano Rosas <faro...@suse.de> > > > --- > > > migration/multifd.c | 67 +++++++++++++++++++++++++-------------------- > > > 1 file changed, 37 insertions(+), 30 deletions(-) > > > > > > diff --git a/migration/multifd.c b/migration/multifd.c > > > index 1851206352..888ac8b05d 100644 > > > --- a/migration/multifd.c > > > +++ b/migration/multifd.c > > > @@ -360,6 +360,11 @@ struct { > > > MultiFDPages_t *pages; > > > /* global number of generated multifd packets */ > > > uint64_t packet_num; > > > + /* > > > + * Synchronization point past which no more channels will be > > > + * created. > > > + */ > > > + QemuSemaphore channels_created; > > > /* send channels ready */ > > > QemuSemaphore channels_ready; > > > /* > > > @@ -561,6 +566,7 @@ void multifd_save_cleanup(void) > > > error_free(local_err); > > > } > > > } > > > + qemu_sem_destroy(&multifd_send_state->channels_created); > > > qemu_sem_destroy(&multifd_send_state->channels_ready); > > > g_free(multifd_send_state->params); > > > multifd_send_state->params = NULL; > > > @@ -787,13 +793,6 @@ static void multifd_tls_outgoing_handshake(QIOTask > > > *task, > > > trace_multifd_tls_outgoing_handshake_error(ioc, > > > error_get_pretty(err)); > > > > > > migrate_set_error(migrate_get_current(), err); > > > - /* > > > - * Error happen, mark multifd_send_thread status as 'quit' although > > > it > > > - * is not created, and then tell who pay attention to me. > > > - */ > > > - p->quit = true; > > > - qemu_sem_post(&multifd_send_state->channels_ready); > > > - qemu_sem_post(&p->sem_sync); > > > error_free(err); > > > } > > > > > > @@ -862,39 +861,37 @@ static bool > > > multifd_channel_connect(MultiFDSendParams *p, > > > return true; > > > } > > > > > > -static void multifd_new_send_channel_cleanup(MultiFDSendParams *p, > > > - QIOChannel *ioc, Error *err) > > > -{ > > > - migrate_set_error(migrate_get_current(), err); > > > - /* Error happen, we need to tell who pay attention to me */ > > > - qemu_sem_post(&multifd_send_state->channels_ready); > > > - qemu_sem_post(&p->sem_sync); > > > - /* > > > - * Although multifd_send_thread is not created, but main migration > > > - * thread need to judge whether it is running, so we need to mark > > > - * its status. > > > - */ > > > - p->quit = true; > > > - object_unref(OBJECT(ioc)); > > > - error_free(err); > > > -} > > > - > > > static void multifd_new_send_channel_async(QIOTask *task, gpointer > > > opaque) > > > { > > > MultiFDSendParams *p = opaque; > > > QIOChannel *ioc = QIO_CHANNEL(qio_task_get_source(task)); > > > Error *local_err = NULL; > > > + bool ret; > > > > > > trace_multifd_new_send_channel_async(p->id); > > > - if (!qio_task_propagate_error(task, &local_err)) { > > > - qio_channel_set_delay(ioc, false); > > > - if (multifd_channel_connect(p, ioc, &local_err)) { > > > - return; > > > - } > > > + > > > + if (qio_task_propagate_error(task, &local_err)) { > > > + ret = false; > > > + goto out; > > > + } > > > + > > > + qio_channel_set_delay(ioc, false); > > > + ret = multifd_channel_connect(p, ioc, &local_err); > > > + > > > +out: > > > + /* > > > + * Here we're not interested whether creation succeeded, only that > > > + * it happened at all. > > > + */ > > > + qemu_sem_post(&multifd_send_state->channels_created); > > > + if (ret) { > > > + return; > > > } > > > > > > trace_multifd_new_send_channel_async_error(p->id, local_err); > > > - multifd_new_send_channel_cleanup(p, ioc, local_err); > > > + migrate_set_error(migrate_get_current(), local_err); > > > + object_unref(OBJECT(ioc)); > > > + error_free(local_err); > > > } > > > > > > static void multifd_new_send_channel_create(gpointer opaque) > > > @@ -918,6 +915,7 @@ bool multifd_save_setup(void) > > > multifd_send_state = g_malloc0(sizeof(*multifd_send_state)); > > > multifd_send_state->params = g_new0(MultiFDSendParams, > > > thread_count); > > > multifd_send_state->pages = multifd_pages_init(page_count); > > > + qemu_sem_init(&multifd_send_state->channels_created, 0); > > > qemu_sem_init(&multifd_send_state->channels_ready, 0); > > > qatomic_set(&multifd_send_state->exiting, 0); > > > multifd_send_state->ops = > > > multifd_ops[migrate_multifd_compression()]; > > > @@ -953,6 +951,15 @@ bool multifd_save_setup(void) > > > multifd_new_send_channel_create(p); > > > } > > > > > > + /* > > > + * Wait until channel creation has started for all channels. The > > > + * creation can still fail, but no more channels will be created > > > + * past this point. > > > + */ > > Let me double check with you here on the TLS use case. > > > > IIUC we still can have more channels to be created if TLS is enabled: we > > notify the sem as long as the handshake thread is created, then the > > handshake thread can further create the tls-armed iochannel? However I > > think I get your point, and that is fine, because if that is the case, even > > though this loop can complete before tls further creates the final channel, > > we'll still see tls_thread_created==true and join() that tls thread first, > > then further we'll join() the next multifd thread even if a new one will > > pop up, or if it failed then nothing to join besides the tls thread. > > > > I'm not sure whether Avihai has any input, I think this can be a good idea > > indeed. > > Nothing special, my understanding of this is the same as yours. > This fix looks solid. > > > there's a dependency chain on the ordering if my above > > undertanding is correct; we may want to document this somewhere, perhaps > > right here on the chaining of threads and how we handle that? > > I agree, this is subtle and may deserve a small note or hint.
IMHO it'll be always better to be verbose on these than "not enough info". One thing I'd also like a comment is now the order is a must to firstly join tls threads then multifd threads, not vice versa, not anymore. We may want a comment above the two join()s there to state this hard requirement. > > > > > This may not allow a concurrent migrate_cancel to respond, but I assume > > this is good enough; the migrate_cancel request is indeed at least so far > > something I made up, but not a request from anyone. We can leave that for > > later and fix the race / crash first. This seems to be a complete fix from > > that regard. > > > > > + for (i = 0; i < thread_count; i++) { > > > + qemu_sem_wait(&multifd_send_state->channels_created); > > > + } > > > + > > > for (i = 0; i < thread_count; i++) { > > > MultiFDSendParams *p = &multifd_send_state->params[i]; > > > > > > -- > > > 2.35.3 > > > > > One other note is I think this will also deserve a cc: stable? But then > > it'll mean all patch 3/4 will also need to copy stable to make Michael's > > life easier. > > > > Let's also copy Dan when repost; after all he more or less owns the TLS > > part. > > > > Thanks! > > > > -- > > Peter Xu > > > -- Peter Xu