* Peter Xu (pet...@redhat.com) wrote:
> 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 <dgilb...@redhat.com>
> Signed-off-by: Peter Xu <pet...@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> ---
> 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;
>
> --
> 2.31.1
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
