If two Linux AIO request completions are fetched in the same
io_getevents() call, QEMU will deadlock if request A's callback waits
for request B to complete using an aio_poll() loop. This was reported
to happen with the mirror blockjob.
This patch moves completion processing into a BH and makes it resumable.
Nested event loops can resume completion processing so that request B
will complete and the deadlock will not occur.
Cc: Kevin Wolf <address@hidden>
Cc: Paolo Bonzini <address@hidden>
Cc: Ming Lei <address@hidden>
Cc: Marcin Gibuła <address@hidden>
Reported-by: Marcin Gibuła <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>