Re: [Qemu-devel] [PATCH for-1.4 stable] block: handle spurious coroutine entries

[Kevin Wolf]

Am 09.02.2013 17:44, schrieb Stefan Hajnoczi:
> bdrv_co_io_em(), bdrv_co_flush(), and bdrv_co_discard() yield the
> coroutine when waiting for aio to complete.  They do not check that the
> request has actually finished.
> 
> In simple cases this works, but it returns early when we get spurious
> wake-ups due to qemu_coroutine_enter() being called from other sources.
> One such example is block-migration.c:process_incoming_migration().

The bug is not in this function but in process_incoming_migration(). It
should never blindly enter a coroutine which is in an unknown state. If
it can reenter here, it can reenter anywhere in block drivers, and
adding a workaround to one place that just yields again if it got an
early reentrance doesn't fix the real bug.

Which is the yield that corresponds to the enter in
enter_migration_coroutine()? We need to add some state that can be used
to make sure the enter happens only for this specific yield.

Kevin