From a5d35dc7e06896a33216610cc86527258735b0e3 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Tue, 14 Mar 2023 16:54:29 -0400 Subject: [PATCH] migration: Wait on preempt channel in preempt thread MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Fixes: 5655aab079 ("migration: Postpone postcopy preempt channel to be after main") Reviewed-by: Daniel P. Berrangé Reviewed-by: Juan Quintela Signed-off-by: Peter Xu Signed-off-by: Juan Quintela --- migration/postcopy-ram.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) 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) {