* Peter Xu (pet...@redhat.com) wrote:
> We have a logic in await_return_path_close_on_source() that we will explicitly
> shutdown the socket when migration encounters errors. However it could be
> racy
> because from_dst_file could have been reset right after checking it but before
> passing it to qemu_file_shutdown() by the rp_thread.
>
> Fix it by shutdown() on the src file instead. Since they must be a pair of
> qemu files, shutdown on either of them will work the same.
>
> Since at it, drop the check for from_dst_file directly, which makes the
> behavior even more predictable.
So while the existing code maybe racy, I'm not sure that this change
keeps the semantics; the channel may well have dup()'d the fd's for the
two directions, and I'm not convinced that a shutdown() on one will
necessarily impact the other; and if the shutdown doesn't happen the
rp_thread might not exit, and we might block on the koin.
Why don't we solve this a different way - how about we move the:
ms->rp_state.from_dst_file = NULL;
qemu_fclose(rp);
out of the source_return_path_thread and put it in
await_return_path_close_on_source, immediately after the join?
Then we *know* that the the rp thread isn't messing with it.
Dave
> Reported-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> Signed-off-by: Peter Xu <pet...@redhat.com>
> ---
> migration/migration.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/migration/migration.c b/migration/migration.c
> index 21b94f75a3..4f48cde796 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2882,12 +2882,15 @@ static int
> await_return_path_close_on_source(MigrationState *ms)
> * rp_thread will exit, however if there's an error we need to cause
> * it to exit.
> */
> - if (qemu_file_get_error(ms->to_dst_file) && ms->rp_state.from_dst_file) {
> + if (qemu_file_get_error(ms->to_dst_file)) {
> /*
> * shutdown(2), if we have it, will cause it to unblock if it's stuck
> - * waiting for the destination.
> + * waiting for the destination. We do shutdown on to_dst_file should
> + * also shutdown the from_dst_file as they're in a pair. We
> explicilty
> + * don't operate on from_dst_file because it's potentially racy
> + * (rp_thread could have reset it in parallel).
> */
> - qemu_file_shutdown(ms->rp_state.from_dst_file);
> + qemu_file_shutdown(ms->to_dst_file);
> mark_source_rp_bad(ms);
> }
> trace_await_return_path_close_on_source_joining();
> --
> 2.31.1
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
