Re: [PATCH 1/6] block/io.c: fix bdrv_child_cb_drained_begin invocations from a coroutine

[Paolo Bonzini]

On 2/14/22 11:27, Emanuele Giuseppe Esposito wrote:
> Anyways, I think that in addition to the fix in this patch, we should
> also fix bdrv_parent_drained_begin_single(poll=true) in
> bdrv_replace_child_noperm, with something similar to what is done in
> bdrv_co_yield_to_drain? ie if we are in coroutine, schedule a BH that
> runs the same logic but in the main loop, but then somehow wait that it
> finishes before continuing?
>
> Alternatively, we would forbid polling in coroutines at all. And the
> only place I can see that is using the drain in coroutine is mirror (see
> below).

I think you should first of all see what breaks if you forbid 
bdrv_replace_child_noperm() from coroutine context.

Drain in coroutines does not poll, it gets out of the coroutine through 
a bottom half before polling.  So if bdrv_replace_child_noperm() doesn't 
require it, polling in coroutines can still be forbidden.

This patch is correct nevertheless.

Paolo