On Tue, Mar 14, 2023 at 12:46:34PM -0400, Peter Xu wrote: > On Tue, Mar 14, 2023 at 10:11:53AM +0000, Dr. David Alan Gilbert wrote: > > OK, I think I kind of see what's happening here, one for Peter Xu. > > If I'm right it's a race something like: > > a) The test harness tells the source it wants to enter postcopy > > b) The harness then waits for the source to stop > > c) ... and the dest to start > > > > It's blocked on one of b&c but can't tell which > > > > d) The main thread in the dest is waiting for the postcopy recovery fd > > to be opened > > e) But I think the source is still trying to send normal precopy RAM > > and perhaps hasn't got around yet to opening that socket yet???? > > f) But I think the dest isn't reading from the main channel at that > > point because of (d) > > I think this analysis is spot on. Thanks Dave! > > Src qemu does this with below order: > > 1. setup preempt channel > 1.1. connect() --> this is done in another thread > 1.2. sem_wait(postcopy_qemufile_src_sem) --> make sure it's created > 2. prepare postcopy package (LISTEN, non-iterable states, ping-3, RUN) > 3. send the package > > So logically the sequence is guaranteed so that when LISTEN cmd is > processed, we should have connect()ed already. > > But I think there's one thing missing on dest.. since the accept() on the > dest node should be run in the main thread, meanwhile the LISTEN cmd is > also processed on the main thread, even if the listening socket is trying > to kick the main thread to do the accept() (so the connection has > established) it won't be able to kick the final accept() as main thread is > waiting in the semaphore. That caused a deadlock. > > A simple fix I can think of is moving the wait channel operation outside > the main thread, e.g. to the preempt thread. > > I've attached that simple fix. Peter, is it easy to verify it? I'm not > sure the reproducability, fine by me too if it's easier to just disable > preempt tests for 8.0 release. > > Thanks, > > -- > Peter Xu
> From 92f2f90d2eb270ca158479bfd9a5a855ec7ddf4d Mon Sep 17 00:00:00 2001 > From: Peter Xu <pet...@redhat.com> > Date: Tue, 14 Mar 2023 12:24:02 -0400 > Subject: [PATCH] migration: Wait on preempt channel in preempt thread > > QEMU main thread will wait until dest preempt channel established during > processing the LISTEN command (within the whole postcopy PACKAGED data), by > waiting on the semaphore postcopy_qemufile_dst_done. > > That's racy, because it's possible that the dest QEMU main thread hasn't > yet accept()ed the new connection when processing the LISTEN event. The > sem_wait() will yield the main thread without being able to run anything > else including the accept() of the new socket, which can cause deadlock > within the main thread. > > To avoid the race, move the "wait channel" from main thread to the preempt > thread right at the start. > > Reported-by: Peter Maydell <peter.mayd...@linaro.org> > Fixes: 5655aab079 ("migration: Postpone postcopy preempt channel to be after > main") > Signed-off-by: Peter Xu <pet...@redhat.com> > --- > migration/postcopy-ram.c | 11 ++++++----- > 1 file changed, 6 insertions(+), 5 deletions(-) Reviewed-by: Daniel P. Berrangé <berra...@redhat.com> This description of the bug & proposed fixed matches what I could infer as the flaw from the stack trace's Peter provided. > > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c > index f54f44d899..41c0713650 100644 > --- a/migration/postcopy-ram.c > +++ b/migration/postcopy-ram.c > @@ -1197,11 +1197,6 @@ int postcopy_ram_incoming_setup(MigrationIncomingState > *mis) > } > > if (migrate_postcopy_preempt()) { > - /* > - * The preempt channel is established in asynchronous way. Wait > - * for its completion. > - */ > - qemu_sem_wait(&mis->postcopy_qemufile_dst_done); > /* > * This thread needs to be created after the temp pages because > * it'll fetch RAM_CHANNEL_POSTCOPY PostcopyTmpPage immediately. > @@ -1668,6 +1663,12 @@ void *postcopy_preempt_thread(void *opaque) > > qemu_sem_post(&mis->thread_sync_sem); > > + /* > + * The preempt channel is established in asynchronous way. Wait > + * for its completion. > + */ > + qemu_sem_wait(&mis->postcopy_qemufile_dst_done); > + > /* Sending RAM_SAVE_FLAG_EOS to terminate this thread */ > qemu_mutex_lock(&mis->postcopy_prio_thread_mutex); > while (1) { > -- > 2.39.1 > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|