From 53021ea1659b8a9074c6f5eb6c65a4e5dddddaec Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Thu, 22 Jul 2021 13:58:37 -0400 Subject: [PATCH] migration: Fix missing join() of rp_thread It's possible that the migration thread skip the join() of the rp_thread in below race and crash on src right at finishing migration: migration_thread rp_thread ---------------- --------- migration_completion() (before rp_thread quits) from_dst_file=NULL [thread got scheduled out] s->rp_state.from_dst_file==NULL (skip join() of rp_thread) migrate_fd_cleanup() qemu_fclose(s->to_dst_file) yank_unregister_instance() assert(yank_find_entry()) <------- crash It could mostly happen with postcopy, but that shouldn't be required, e.g., I think it could also trigger with MIGRATION_CAPABILITY_RETURN_PATH set. It's suspected that above race could be the root cause of a recent (but rare) migration-test break reported by either Dave or PMM: https://lore.kernel.org/qemu-devel/YPamXAHwan%2FPPXLf@work-vm/ The issue is: from_dst_file is reset in the rp_thread, so if the thread reset it to NULL fast enough then the migration thread will assume there's no rp_thread at all. This could potentially cause more severe issue (e.g. crash) after the yank code. Fix it by using a boolean to keep "whether we've created rp_thread". Cc: Dr. David Alan Gilbert Reviewed-by: Dr. David Alan Gilbert Signed-off-by: Peter Xu Message-Id: <20210722175841.938739-2-peterx@redhat.com> Reviewed-by: Lukas Straub Signed-off-by: Dr. David Alan Gilbert --- migration/migration.c | 4 +++- migration/migration.h | 7 +++++++ 2 files changed, 10 insertions(+), 1 deletion(-) diff --git a/migration/migration.c b/migration/migration.c index 2d306582eb..21b94f75a3 100644 --- a/migration/migration.c +++ b/migration/migration.c @@ -2867,6 +2867,7 @@ static int open_return_path_on_source(MigrationState *ms, qemu_thread_create(&ms->rp_state.rp_thread, "return path", source_return_path_thread, ms, QEMU_THREAD_JOINABLE); + ms->rp_state.rp_thread_created = true; trace_open_return_path_on_source_continue(); @@ -2891,6 +2892,7 @@ static int await_return_path_close_on_source(MigrationState *ms) } trace_await_return_path_close_on_source_joining(); qemu_thread_join(&ms->rp_state.rp_thread); + ms->rp_state.rp_thread_created = false; trace_await_return_path_close_on_source_close(); return ms->rp_state.error; } @@ -3170,7 +3172,7 @@ static void migration_completion(MigrationState *s) * it will wait for the destination to send it's status in * a SHUT command). */ - if (s->rp_state.from_dst_file) { + if (s->rp_state.rp_thread_created) { int rp_error; trace_migration_return_path_end_before(); rp_error = await_return_path_close_on_source(s); diff --git a/migration/migration.h b/migration/migration.h index 2ebb740dfa..c302879fad 100644 --- a/migration/migration.h +++ b/migration/migration.h @@ -195,6 +195,13 @@ struct MigrationState { QEMUFile *from_dst_file; QemuThread rp_thread; bool error; + /* + * We can also check non-zero of rp_thread, but there's no "official" + * way to do this, so this bool makes it slightly more elegant. + * Checking from_dst_file for this is racy because from_dst_file will + * be cleared in the rp_thread! + */ + bool rp_thread_created; QemuSemaphore rp_sem; } rp_state; -- 2.11.0