Re: [PATCH] migration: Wait on preempt channel in preempt thread

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> 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.maydell@linaro.org>
> Fixes: 5655aab079 ("migration: Postpone postcopy preempt channel to be after 
> main")
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

queued.